More power for moderate listening levels?

Brian, I think you answered your own question. If you are straining at times you either need more power, more efficient speakers or add subwoofers and a 2 way crossover. 
I am of the camp that there is no such thing as too much power. XA100.8s would be the minimum IMHO. For that money I would get JC1+'s. Good high powered amps have an effortlessness missing in smaller amps(unless you have very efficient speakers.) Before you notice the harshness the amp is already clipping on transients "stressing the sound."  I tend to go for speakers on the less efficient side, 89dB or so and would never consider an amp less than 200 watts. I also have a tendency to gravitate towards class A amplifiers and I also use subwoofers crossed at 125dB which increases the headroom of my system 10 dB. With all this I can get 110 dB out of ESLs. More than enough to give you a headache. 
With your system I would (and this is me) get a Trinnov Amethyst preamp, XA 100.8's and at least two passive subs. With the Harbeths this would give you a SOTA point source system for sure which should do an easy 105 dB probably more. It would be totally effortless at lower levels and for sure put a big smile on your face whenever you light it up.

Mike

What you are looking for is more of what the XA25 delivers, power and great sound. 

Much to sort through here. I thought the thread might die after a few posts but the community here really came through. Thanks all. 

I owned the Harbeth SHL5+ for many years and they sound great at moderate volume as well as loud volume. I bought the Harbeth's because I sold my house and they played well at lower volumes unlike the Vandersteen Quatro's I had before.

I successfully used an Ayre AX-7e in combination with an Ayre Codex and in my 15 x 21 x 9 room I could not crank the volume loud enough to cause any distortion. The Ayre is only 60 watts and still drove the SHL5+ well. I upgraded to the Aesthetix Mimas integrated, which puts out 150 watts a channel, and that was even better. So I am not sure that more power is the answer you are looking for. Alan Shaw says that any 100 watt+ amp should work on any of his speakers. I would suggest trying a few different amps to see what works best with the Harbeth's in your room.

I did try the Pass 250.5 amplifier along with the Audio Research LS26 preamp and I did not like it with the Harbeth's. Nothing against Pass, just not my cup of tea. Sounded great with my Quatro's though.

Good Luck!

Using an SPL calculator you should be able to achieve 97 to 100dB levels at your normal listening position.  The Pass amp is rated at 50 watts peak power.  Personal taste differ and some would not consider this level very loud.  I suggest you use an SPL meter to gauge how loud you listen and at what point your system starts to sound strained.