SET 45 and their (real) required speaker efficiency

Most 45 SET amps are about 1.5 watts output and 2 watts is really stretching it. Distortion of the amps gets very high near their max power. So really, with a 45 amp, you want to not exceed 1w and preferable at 0.5 watt or less.
94.5dB even with 8 ohm and impedance friendly speakers are not efficient enough for your 45 SET amp to do serious listening. Can it be satisfying? Of course. Likely in a small room and with small ensemble classical music, you're listening at between 85-88db. The question for amp/speaker matching is really about headroom.
Here is how you can look at it. for every 3dB of speaker output, you will need to double the amp power e.g. if your speaker is 94dB/1watt/1meter efficiency.  Then for 97db, you need 2watt. for 100db, you will need 4watt. For 103db, you will need 8 watt.
Since you amp is only 2 watt, them your Omega speakers can only hit 97.5dB max and that's almost at clipping level. so your effective listening range is between 85-97dB i.e. 12dB of headroom.
But imagine if you have a 100dB speaker. Then your 2watt amp can generate 103db i.e. a listening range from 85db to 103 db which is 18db headroom. This is 6dB more than your existing Omega speaker i.e. 4 times more power.
So really, what it boils down to is that 45 SET amps should ideally be matched with speakers with a minimum of 100dB (in the correct impedance as your amp out impedance). Just my 2 cents.

I have three SE amps: a 45, a 2A3 and a 6BG6. 1.75, 3.5 and 6 watts respectively. And a pair of Heresy's. The 6BG6 has enough (barely) to work. The other two are only good for headphones. 

Hey,

In looking around, I keep checking out Decewares' site to compare how their rated efficient speakers stand next to others. I've been looking at a lot of Lowther cabs. I guess at this point I should move this over to the speaker section, but since you guys are here I'd like to throw out a quick question regarding one speaker on Decs site. I ran across a small read regarding how new technologies are allowing for better sounding larger drivers. I then remembered a 15" 100db driver over at Deceware. It's the Model: ZF15L (open baffle)
Have any of you run across anyone that has perhaps heard this speaker? And as a general question, can speakers at Decs site be considered good enough to stand on their own sold at Deceware or elseware? I can almost answer this question as they have an Omega speaker. I think I've scratched anything under 98db off my list, even though I'm listening @4' to tweeter and under 90db 90% of the time with solo to small ensemble classical. Don't shoot me, but I'll leave a YouTube link to the speaker. What got me curious was the piano and cello sound. Both of these instruments of course sound very large, and they seemed to sound just about good enough in this demo. Speaker tone, and all the rest aside. 
https://www.youtube.com/watch?v=byhqlewpQQY

thanks

VVCV, There are several factors that determine the speaker sensitivity needed to use 45 tube amps. Many have already been stated. Room size, music density, max SPLs for dynamic peaks, average listening volume, etc.Keep in mind that many manufacturer's speaker sensitivity ratings are for one central frequency. As ddriveman suggested the lack of headroom for dynamic peaks is often the limiting factor. Speakers without crossovers can help. Passive crossover can sometimes be a more difficult load and can consume power. A flat impedance curve and minimal phase anomalies help.

I use mostly low power SET amps to demo our speakers which have a sensitivity of 105 dB. We have two pair of 45 mono amps, a pair of 2A3 mono amps, a pair of 300B mono amps and a few other amps of various types. Our showroom is 21 feet by 31 feet with a ceiling that starts at 9 feet high and goes to about 14 high feet at the rear. Our speakers have separate powered woofers so the SET amps don't have low bass duty. The main towers go down to about 100 Hz but we typically set the active crossovers to 120 Hz. The amps are directly coupled (no passive crossovers) to the drivers. The 45 tube amps, to me, sound the best and have no trouble driving the towers to high volumes with no compression. In our setup the 2A3 amps are a close second. Our speakers are a line source so the dB loss per distance is less than a point source speaker which helps. 

Our speakers are unique in many ways so maybe not the best example but typically for a medium size room, listening to reasonable/enjoyable volumes a minimum sensitivity of 100 dB should work. With so many variables you should audition the potential combos.

@vvcv  A nice plan B to consider:

If you are set on the type 45 power tube, why not get an amp that uses a pair in push-pull? I've done direct comparisons; the thing about any SET is that you really don't want to run them past about 20% of their total output power, since higher ordered harmonics start showing up on the transients (where the power is). This causes the amp to sound 'dynamic' as the ear uses the higher ordered harmonics to sense sound pressure- its distortion masquerading as 'dynamics'.

But with the same tube operating in push-pull, but otherwise class A and zero feedback, the total power will be more like 6-7 watts and about 90% of that power will be usable as opposed to 20%. So you will have a much wider range of speakers you can play.

Now to the comparison; side by side it was obvious that the push-pull amp using 45s was more detailed (vocals were easier to make out) and had more bandwidth both in the bass and the highs and every bit as smooth. IOW, no downside except you need a pair of power tubes per channel.