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.
Post removed 
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.
Must do the audition in your system, so get a dealer loan. ake a look at Cayin, great sound and value.
I just paid 450 for a Jolida FX-10 12 watt amp for my office. Sounds nice. Way to dip your toe in the tube ocean.