Guy, unless things have changed in your rig, AFAIK, you have two sets of speakers, the D'Appolito ones and the Tympanis. If that is to remain, then the 15-18 watts probably won't do it, surely not for the Tympanis. Since changing from about 35WPC to 80 WPC on my Tympanis, above 360Hz, the macrodynamics and microdynamics were dramatically improved. So, I'd suggest that anything under 60WPC or so is a non-starter. Think KT88, transmitter or sweep tubes.
The major aspect of the sound of my system that has bothered me is the screechy treble. It seems each time I improved some other aspect of the system, the screech has gotten worse. Mostly on horn peaks or other high power high frequency sounds. I figured that like Accustats and ET speakers, perhaps this is a planar/ribbon/'stat issue. It is a challenge for lots of folks and I certainly know of some guys who won't use those speakers because of that sound and the power requirements.
Now, reading more about the PowerDrive (source follower) driver stage and the various forms of distortion resulting from RC coupling at the output grid, When I built an 18 watt SE SV572/100 amp for a friend, I used a source follower to drive the grid of the 572, which needs lots of current in A2. Tried it in my system. Guess what? Even with just 18 clean watts, no screech! Incredible treble resolution. Great sound. BUT, definitely lacking power for dynamic music at louder levels. Having heard the 572 amp and Dave M.'s Powerdrive amp, I really think what I was hearing with other amps, and what I hear in some other systems, is distortion due to the RC coupling. Tubelab George really explained a lot when he listed the various forms of distortion emanating from this scheme. Not just blocking, but in feedback amps, the distortion resulting from NFB attempting to "correct" gain lost at clipping, making the output even worse. Is this another reason we in general prefer amps without much overall NFB? Maybe.
So having said that, and having read extensively on PP amps with decent power, I'd suggest the amp build, customized for you of course, suggested on diyaudio, one which Tubelab George obtained ~60WPC from a single pair of KT88s using a source follower driver in TRIODE Class A2! That is an amp worth building.
should suit your needs. All the gain needed from two DC coupled 6SN7 cascaded differential amps, that feeding the MOSFETs. Neat method for DC balance is incorporated. Try it with the ST70 or Fisher 500 OPTs. They will be fine for power over 250HZ. If it works exceptionally, redo it using the KT550 transformers.
The schematic, with 6L6GC's is in post 460.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-46.htmlCheck out post 272 for results without NFB.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-28.html#post1883120