Sure, I've preferred the characteristics of one over another, and made my choice as to what to own, but have never found myself unable to dial up a professional sound.
The OP it seems, asked what the differences between GK and Ampeg might be.
The best advice by far was to ignore all opinons and make a choice as to which one sounded best for him, by playing through them.
Seemed simple enough.
But then, there are those who...............
Point understood. But a thread like this also provides an opportunity for suggestions on how to make the most of trying out a new amp.
Comments on tone aside... it still helps to give the OP an idea of what to expect or what the best way would be to approach the controls. GK's voicing is naturally brighter by default when all the controls are flat, for example.
When I walked into Guitar Center for the first time, I went straight for the SVT-CL to give it a try. I really liked the sound (not the price tag

), but if someone had told me how good an overdriven poweramp tube could sound at the time, I'd have probably walked out with a bigger smile on my face.
But see, I didn't know that at the time - I doubt most beginning bassists would know that cranking the Master volume and using the Gain knob for volume control instead can be a good thing on a tube amp, and get you a different sound. Now, whenever I get a chance to use one, I have a better understanding of how to make the most of an Ampeg all-tube head for different tones.
The same sort of thing applies even to many pro bassists. If you've spent a lot of time with hybrid and all-tube amps, you'll probably look at SS amps like GK for the first time and think "no, I don't want to push that preamp or poweramp too hard", because the stereotypes of SS amps that sound bad when pushed or overdriven have never gone away. Of course, by "you", I mean the hypothetical "you" - a made-up (but common) person who is more familiar with the stereotypes than the realities of solid-state amps.
In the end, it does narrow down to his own trial run of a GK amp. But if he were me, I'd definitely appreciate a few pointers.
And if someone goes into a thread expecting purely unbiased information, then the expectation may be too high. I think that if a Thunderfunk or Ashdown fan walked into this thread, you'd be more likely to get a nonbiased answer. But I don't see either of those brands in the thread title. Who do you think it's more likely to attract, then? The GK and Ampeg fans, of course.
Take these comments - as well as all the others - with a grain of salt. That's my rule of thumb for navigating opinion threads.