Probably the one important weakness of Plinius amps is its high frequency extension which is relatively rolled off/muted, so you do not really hear the crispy sparkly sound of live instruments.
The Ulta Upgrade from Vince Galbo, former Plinius importer and current warranty tech, does much to fix this fault. My SA-103 sounded a little plummy in the midbass and, as Pani says, rolled in the highs. After the $1,500 upgrade, which replaces all caps and resistors and beefs up the power supply, among other things, it became far more balanced through the frequencies and extended at both ends, with more nuance and micro-detail. Quieter, too. Now it is an excellent amp, IMO. While not quite as "sparkly" on top as my VAC 300.1a, it's close, and that says a great deal.