Sai,

Congratulations!

If you are referencing the switch in the front center. That changes the bias for the input tube. Leave the amp on. Go ahead and switch it while it's playing. You will hear a pop through the speakers. the meters will dance and the VR tubes will blink. This is fine.

There is no right or wrong try both and see which you enjoy more.

I prefer it towards the back with my PAP quintet.

Also try switching the Hi/Low switches on the back. They should both be in the same position. This is the hi or low ohm setting for speakers. Try both and see which you prefer.

Seems I jumped the gun a little on the bias switch. Before 3 days ago it's been 2 years since I had the UFO25TH in my system. Now it's a completely different system.

Last night I ended up with the bias switch toward the front of the amp. With the switch in the back position the bass was a little bloated. Currently I'm running an OC3 tube in the front. I have an OD3 on order. Running that tube, I might prefer the switch in the back position.

Interesting, I am using the OC3/VR105 from Steve. And having it switched back gave the sound more energy overall. Not just in the bass department, but mids and treble as well. I need to try the other 2 tubes that Steve provided in there.

Also, one quick comparison between the Torii MKV and SE84UFO25. At the exact same preamp volume and all else being the same. The dB levels were different (as expected) on Mark Knopfler’s Prairie Wedding. Sure I’m not being super scientific in the measurement, but neither is my listening.

Torii: Min - 63 dB, Max - 84 dB, Avg 71 dB
SE84UFO25: Min - 67 dB, Max - 77 dB, Avg 71 dB

SE84UFO25 in my setup is plenty loud!

Another observation. My ear tells the same story as your measurements. My findings are I listen at lower averages and peaks when the MKIV is in use. It seems louder though.

Comfortable, exciting listening levels.

MKIV peaks 72-76dB

UFO25TH peak 78-82dB

Seems there is more space between the notes on the MKIV.

Great points:
- on the speaker differences. In stark contrast, the Zus are a full range 12” driver with a tweeter that only kicks in above 12 kHz.

- The wider range in dB levels on the MK V do have the effect of creating more space between notes. The SE84UFO25 does sound a lot more precise/concise.

Another note, I do feel like the SE84UFO25 in my setup has a soundstage that is smaller than the MKV but with considerably more depth. So more 3D than the MK V.

Steve, I prefer a more euphoric sound. The ZROCK2 is at 12-2 o’clock for the euphoric and harmonic magic it brings. With the OA3 in the front and no ZROCK2, it seemed like the ZROCK2 was in and turned up a lot. Eight 15” drivers change the experience. With the OA3, it could be too much energy in the room. Over 78-80dB, things get out of control exponentially. Peaks of 75dB are exciting and immersive.

When I first had the UFO, I was using an OA3 in the front position. OC3 was better. This morning received a RCA OD3. This is significantly better than the OA3. The slow and syrupyness is gone. Which could be due to room overload even at lower volumes. With the bias switch towards the back, it is very similar to the Torii. Torii is still snappier. Torii has a bit more shimmering highs and the bass hits are tighter. The amount of control the UFO is now demonstrating is truly impressive! 

Torii MKIV tubes:

EML 5u4g
Westing House OA3
RCA 0c2
GL KT-77
EH Gold Pin 6922

UFO25TH tubes:
Sophia Electric 274b
Westinghouse OA3
RCA OD3 Bias switch towards back
6P15P-EV
EH Gold Pin 6922

I am curious to try an EML 5u4g in the UFO25TH. Not wanting to run one of the matched EML 5u4g I’m using in the Torii, Maybe I’m a touch neurotic with this stuff. I wonder if some of the density differences are because of the rectifier differences. The UFO25TH has Jupiter caps while the Torii MKIV has Miflex caps. This could account for some differences as well. 

First track: Michael E - Beauty & Seduction. 25 seconds in a bass hit comes in that continues throughout. The Torii feels like it’s trying to stop my heart! It’s so sudden and fast I’m concerned it’s hurting the speakers. UFO25TH does an excellent job keeping up. If I hadn’t experienced what it was like with the Torii MKIV, I wouldn’t question the tightness of the bass hit with the UFO25TH.

Second track: Martins Garden - Paladin’s coming. Both amps the room disappears, and it’s like floating in space surrounded by music. The UFO25TH doesn’t have the distinct texture on the bass like the Torii MKIV. The UFO25TH is smoother and creamy on the top. All the information is there. The Torii MKIV has a bit more separation between everything. 

Third Track: Slenderbodies - Anemone. Both amps are very close. The UFO25TH has a bit more density than the Torii MKIV. Changing a cable or two may get them extremely close. This also could be the difference between the Jupiter and Miflex caps. 

Fourth Track: Lhasa De Sela - My Name. The rumbling bass that rolls around the room is more defined with the Torii MKIV, more texture. 

Fifth Track: Michael Nesmith - Cruisin’. Both amps rock! I’m having fun. UFO25TH 90+dB everything is tight and clear.  The Torii will play louder…I have no desire to go any louder. (Sorry not available on Qobuz or Tidal I selected some songs from my library. I then created a streaming playlist.)

https://open.qobuz.com/playlist/9122596

https://tidal.com/browse/playlist/f2112e8f-b0ee-44e2-9be4-a9024464b6ef

Both amps sound very similar now. Steve say’s he voices all his amps to sound like the UFO and he has succeeded. Speaker choice and tube selection can radically change how the amps sound. Learning how the amps respond to tubes and the way you want to change the sound, you can take the amps from fast and clean to molasses syrup.

On another forum, I saw that someone had a Torii Jr. for less than a month before selling it off. He didn’t like the sound. This seems like a huge misstep, as he didn’t even give himself the chance to explore the possibilities.

I have much more experience with the Torii MKIV. Over the past year, I’ve tuned it to my system and the sound I enjoy. Having the UFO25TH for 2 weeks and getting them to sound almost the same is really rewarding and an excellent learning experience. I still find that I listen at a slightly lower volume (3-5dB) with the Torii MKIV.

I still prefer the Push-Pull sound over SET. The differences are so much closer than my last experience. The Torii has more options to fine tune the sound to exactly what one wants.