I have OTL mono amps. They're from Transcendent Sound right where I live in K.C. Mo. They are the T-16 mono kits developed by Bruce Rozenblit. Each amp can put out 40 watts of very high quality sound. They can drive a 4 ohm load, which they did quite well when I first got them. But eventually and intentionally I mated them with your Theater 4 Pi's, and I got to tell you the resulting offspring between these two designs are pure joy. Bass is dynamite, mid's & highs very realistic. Also I'd built Rozenblit's Grounded Grid preamp from his book Audio Reality and this as much as everything else makes me happy I persued this audio mania. And you know how insane audio can be. I built it me self and saved a heap. The OTL's downside is by design, HEAT! But Rozenblit's design philosophy is geared toward that issue, and reliability. So even with 16 russian 6c19pi tubes bristling on top each amp, they're not space heaters in the winter and weber grills in the summer. Plus I've never had a lick of trouble except one output tube that the heater element went bad. Luckily I found also a reliable and inexpensive source of tubes for this amp on e-bay, called KW tubes in Lithuania. The thought of re-tubing these amps eventually with 32 tubes was daunting to think of. But at a little over 3 bucks apiece, thats real good, about 1/3 what I'd searched elsewhere.