not to be technical and anal-retentive, but "groove tubes" don't sound bad. blame the manufacturer whose tube it is being relabeled as GT. now, the stock groove tubes in amps are typically sovtek brand, which tend to have a pronounced high end, not so much in a positive way at that. many people don't care for that much end when the tube gets cooking and overdriven. it can sound brittle, sometimes thin, ESPECIALLY in a traditionally-voiced, mid-scooped fender amp. the high end might fare better in darker amps, maybe even some marshalls, etc.
i actually don't mind their sound as much as others, i can deal with it, but i agree that JJ's are better any day. i don't know, not going to get into my favorite flavor of tubes right now.
groove tubes also relabels mullard and JJ brand tubes, but you dont see those everywhere.
groove tubes gets a bad rap because many of us are beginning to think it's a bunch of hoo-hah, paying that extra cash for supposedly failproof tubes, that are supposedly put through extra testing, which may not be so true. GT's have a tendency to do just that, FAIL. again, tubes are unpredictable, and inconsistent, and can fail anytime, but the GT thing is that you're supposed to be getting certified, guaranteed performance, which we all know can't actually happen. its a waste of money basically.
to answer your question, i think alot of people are forced to take a gamble and buy amps online that they've never seen in person (well, in their possession, or at their mercy with guitar in hand). so basically, the only hearing trhey have done is on youtube, or maybe a friend has the same amp. see, alot of the "prized" amps today are from back in the day and frankly aren't commonly seen in stores anymore. plus, consider the fact that businesses are moving to the internet slowly because its more cost-effective. it costs a business way less $$$ to run one warehouse (or 2) and a website, than it does to run a supply chain or franchise. i think if it were still pre-1990, it'd be a different story, about buying amps in peerson vs online and relying on youtube.
i just recently bought a Pro Jr, but theres only like one or two stores within a 30 min. radius that have one on the floor. needless to say that helped my decision. i'm very weary about buying without trying, so that was good for me to have found a store with that amp. in-store stock is very limited, again, because it's less costly to manage small inventory. consider the fact that major chains like GC are actually carrying mainly new stuff. they aren't selling any vibrolux's anymore. i guess it's all about the modeling amps now, and trying to sell those amps that can "do it all". something like a DSP or GDEC.
another thing, you can only do so much when you test an amp. i feel kind of awkward in a small quiet store if i were to crank up a tube amp and let it sing. nobody really feels like being blasted through the window from that wall of sound. you can imagine my test for the aforementioned Pro Jr wasn't much of a test. i liked it at low volume, but had to take a chance and trust that everyoine was right about its pure, unadulterated el84 tone when driven. even then i was in the same situation as everyone else buying on MF or craig's.
how about guitar pickups? what a gamble that is even. you can't testdrive pickups. you HAVE to go online, there's practically no way around it.
anyway, you see my point. there's little options available today. and it sucks, but we can't do anything about it. so people go through exhausting amounts of research to find the right amp, and they hit up musicians friend, and switch out the tubes, speaker, caps, maybe even re-wire it to be p2p, probably change the transformer.....by now, you have to wonder why they didn't just BUILD their own amp.