I'd love to see a schematic for your amp. Small amps can be absolutely amongst the best sounding ones. Years ago I remember talking to Mr. Atarashi on the phone and he told me that one of the most beautiful amps he had ever built was a 71A PP amp for like 1.5 watts output.
the exo-100's cobalts are long ago sold out. But we can make the EXO-100's with other cool mats like high or low nickel, M4, M3, pinstriped, etc. At small power level of 500mw a lot of lams would do quite well.
best to contact me by phone (evenings best) at 215 288 4816 or by email at acrosound at aol dot com
Mike