I run a Aesthetix Calypso Signature into a Pass Labs x350.8 about half the time when i want to go 'warm'. I switch that out between the xp-30 as my silly whims change month over month. Plenty of gain, no compatibility issues, and is super sweet. I usually run early 50s mullard or 60s amperex to adjust the 'warmness' in both the 12ax7 and 6dj8 positions, although OEM tubes work just fine. Playback is going through some Wilson DAWs. It's great too since it has HT passthrough and doesn't need to be on to watch TV or do movies. Plenty of inputs and outputs too....
