I'm no technical expert but I know one of the two choices will sound better...more dynamic.  Since your speakers are right in between, I doubt their is a right answer.  Your ears will have to tell you which is right for you.  Mark.

so the difference i'll notice is in dynamics?

think i got it figured out.  i have the impedance switch set for the lower ohms and the bass switches off.  now my question is about the treble adjustments.  which way is off?  also the Reconstructive Feedback Switch - is this ONLY to remedy whistling or does this affect the sound as well?  it's hard for me to tell...thanks for your info guys.

Mark I don't know what the teble cut is, but I think it's more comprehensive than the Mk IV--you can I think completely remove the treble.

Ro,  I haven't messed with this control much.  I have turned it on and listened for a few minutes then turned it off.  I think it's supposed to change the sound stage...make it deeper/wider? Below is what's written in the manual...it doesn't tell us what it does though.  Mark.

"Reconstructive Feedback Switch - Whistling Sound
This switch can be operated “on the fly” to turn on or off the reconstructive (positive) feedback. A rare feature to have on any amplifier because it is so difficult to define a window of stability that works on everyone’s speakers. Therefore, we do not claim this feature will work with everyone’s speakers. You’ll know if this feature is compatible with your speakers if you don’t hear the amp start to whistle when the switch is ON.
The reconstructive feedback switch has an ON and OFF. When facing the front of the amplifier, reaching over it and flipping the switch towards the back of the amplifier is ON, while the switch pointing towards the front of the amplifier is OFF. It is important to keep this switch off any time speakers are disconnected as damage could occur.
If you hear the amplifier start to whistle, turn the reconstructive feedback switch OFF until the cause has been corrected. While we don’t anticipate many if any speaker incompatibilities, if you have a pair of speakers that are not compatible with this feature just contact us and we can discuss one of several possible solutions."

https://www.decware.com/newsite/TORIIMK4.pdf

thanks guys!

I agree with all that the adjustments are about adjusting to room, system and tastes. I run mine with the treble knob higher (clockwise) than straight up most of the time, depends on the tubes. The bass switch I like off, mostly because I like how the mids are richer. With the Tungsol KT66 I like the impedance switch at 4 ohms (back), but this is my speaker rating too. With bassier power tubes I have liked it at the eight ohm setting. I did not like the Reconstructive Feedback for quite a while of the burnin finding it too diffused, lacking focus. This really changed at some point. The focus came in in a very musical way but also a really nice dose of spacial information and more complex edges. The soundstage is quite saturated while micro detail is more of the balance, more air, textures, and showing more of the space around each player as well as wider room ambient information. I can't recall how many hours it took for this to happen, but it was a pleasant surprise! I find it a little hard without it now. Actually, this has become a highlight of the amp in my system and room!

Hi Roggae,

My amp has the stepped attenuator .

In my setup, the max volume I run the amp is at the halfway point, 10 out of 20 clicks.

In such a situation, the RFS setting makes the sound diffused.

I don't know what Reconstructive Feedback does technically, but I can say what I hear in this room. This system/room, once adjusted, has great frequency balance and a live character. It has excellent complex detail throughout, with rich, tight bass that you feel, to very nice ambient information. It reveals pretty much everything and with a good soundstage.

I had not noticed this before since all the tube sets I have used have had very good complete, textural highs, and a nice openness. This is partly from my general preference for OB3s over OA3s, giving less density and more clean space to the sound. Now, with green (about 35 hours) Valve Art 350Bs in, they sounded too rigid and cool at first, so I replaced the quite open Ultron 6BQ7As with warmer RTC/Mullard E188CCs in the power positions of the CSP3 to richen things up. This has given me a very good sound for burning in the new power tubes.

When I checked it today, the effect from reconstructive feedback was there, but more subtle than I had previously heard. This is the first I have noticed the circuit depending this much on tubes. The 350Bs, having become more complex, smoother, and warmer I put the Ultrons back in the CSP. This opened and clarified things enough that the Reconstructive Feedback circuit became what I was accustomed to...a much more notable difference on and off.

Similarly, with the circuit depending on complex detail being there to begin via gear, cables, tubes, room, etc...also recordings with more openness, detail and ambience show it more. Either way it is relatively subtle in some ways, but really kind of big in others in terms of the overall feel of the music.
 
Also, if I recall correctly, it was around 350-400 hours before I started to really like it. For a long time I kept it switched off, preferring the more focused sound. But finally burnin made its previously too diffuse sound tighter, clearer, and quite tasteful in my system. Now it stays on.

The recordings with a good soundstage here are generally similarly saturated both ways, wide and deep, but the sound with reconstructive feedback is different, increasing subtle information. From note hits to fading trails, it feels sort of like it takes bits of information and divides them into more, smaller bits. The increased senses of fine detail and resolution make tones and edges smoother, more liquid and more complex at the same time. It shows best in edges, in air, and in ambience close to the instruments, and beyond, but I think is throughout. The space between and around the instruments is richer in ambient information giving a nice atmospheric quality. It also helps recording ambience seamlessly integrate with my room ambience.

Finally, I have not fully explored this, but I have noticed a number of times that reconstructive feedback can give mixes with less focus on soundstage more soundstage spaciousness and definition in this system.

how do we experience negative feedback as the listener?