Hi, I need some assistance. I'm trying to figure out what would be a good SET match for my ZOB speakers.

I have a Rega Apollo CD player which I've read has 2.175 volt RMS output.

http://www.stereophile.com/content/rega-apollo-cd-player-measurements#YZbZaGDrU5...

The amp I use is rated at 20WPC into 8ohms maximum, and has an input sensitivity of (I think) 600mV. With my preamp set at -15db on the volume control, I can drive the speakers to about 86db average with 94db peaks (based on a free iPhone SPL meter app), which is quite a bit louder than my average listening level.

Is it correct to calculate the amplifier's voltage gain in this way:

http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm

power in Watts = output in volts^2/speaker nominal resistance in ohms
ZOBs with FRX2 drivers are rated 8ohms, and
Sugden A21ap amp is rated 20WPC into 8ohms, so at 600mV input:
20 = V^2/8
160 = V^2
V = 12.649V

And, using this:

http://eaw.com/docs/6_Technical_Information/StudyHall_and_TechNotes/dB_calculati...

My preamp is attenuating the Rega from 2.175V to an amplifier input voltage of:

Volts(input) = 10^(dB/20) x volts@0dBu (0.775)
V(i) = 10^(-15/20) x 0.775
V(i) = 10^(-0.75) x 0.775
V(i) = 0.137817 or 137 mV

If the amp's gain is linear, then I am using 137/600mV to hit a quite loud listening volume. I know an amp needs some healthy headroom, but does this mean that I am effectively "using":

output voltage at very high listening levels = 137/600 * 12.649V
V(o) = 2.888V

Say we give that value a healthy 100% headroom:

2.888*2 = 5.776 V(o)

and therefore a suitable SET amp power rating would be:

W(set) = V^2/R
W(set) = 5.776^2/8
W(set) = 4.17

So, a ~4 watt SET would be enough to drive these speakers to absolutely ridiculous volume? Does that all add up? That seems way too low, but if it's right, then I could choose say an 801A or 300B output tube with an appropriate driver, and stay out of 845/GM70 territory, which would be extreme overkill.

Right, or wrong?

Tom, I think all the Decware Amps will work with a 94 dB sensitivity speaker.  I have a pair of 2.3 WPC Zen Signature Monoblocks that work with Omega Alinco 7XRS speakers rated at 93 dB that give me more than enough volume.  I like my Zu Audio "Soul" speakers better...rated at 98 or 99 dB. Both @ 8 ohm. Mark.

PS...Tom, I should add that the Taboo MK III is a great Amp for my Audeze  LCD 2 headphones but doesn't drive speakers well for some reason.  The Omegas were lacking in usable volume and I can't say I liked how it drove my Souls either.  For me the Taboo MK III (4.6 WPC) is strictly a headphone amp.  With that exception all Decware Amps should work.  A phone call to Steve would confirm this.  Mark.