in general you can expect better mid-range, a more natural sound, possibly better soundstage - and some 'rolloff' in the bass. if you want to hear what they do well, bring mid-range heavy (voices, jazz ensembles, acoustic instruments). if you want to hear their weaker side, bring your bass-heavy recordings.
Auditioning a Tube Amp
I've never heard tubes and am seeking my first purchase. I'm going to be buying local and auditioning. What should I be looking and listening for to ensure everything is on the up and up? I'm leaning towards something newer like the Sophia Baby so as to not end up with a "project." Any specific tips appreciated.