I just picked up one of these things last week and I have been enjoying the heck out of it. I got it off ebay for around $50. I planned on changing the tubes but when I got it, but it already had a Ei 12ax7 in slot1 and a Electro Harmonix 12AT7 in slot2.
I've been through my share of pres and recently came to the realization that 90% of the time my EQ was flat since I do most my tone shaping from my bass. So this was right up my alley.
The clean sounds remind me of my old presonus tubepre (RIP) w tungsol. I would describe the clean tone are round and fat but it also has the ability to get nice and hairy and wooly sounding. I know these units aren't made for overdrive but I went ahead a pushed it just to see. Sounds bad not as bad as the presonus did but still.
The compressor can go from smooth and unnoticeable to hard wall type effects compression.
Overall I think this is a great little unit. It can be a tad hissy for solo bass tracks as you will hear from the recording, but in any type of mix it would be unnoticed.
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Art Tube Pac review w clip. 
