I am an amp tech, and a bass player. A "good amp" is pretty subjective. It has to have sufficient power and headroom. It has to be reliable and consistent. The preamp has to be as versatile enough to meet the player's needs. Each company has a "signature" tone and feel. My GK1001RB was a "good amp" but I never really liked it's signature. The low end was tight but just didn't sound "right" to me. I now use a PF500. Reliability is an issue here, but I do love it's signature tone. I gigged with an SVT in the mid 70's to the early 80's. The PF500 tonally takes me back there.
I've got 30 year old Peavey bass heads that are still pretty good amps. My Acoustic 370 is a "good" amp, and I love it's sound. It's pretty low focused and has a very large, uncontrolled low end, compared to modern amps, but it is great fun. That Acoustic had problematic edge connectors, and mine were taken off and hard wired decades ago. Peavey's had problematic molex connectors that intermittently failed over the years.
Tube amps, whether point to point (rare), eyelet boards (old Fenders) or PCB will need maintenance over time. Think of a car. No matter how good, periodic maintenance is necessary. I've gigged for years with tube amps. I ALWAYS had a backup along. I still carry a backup.
A "good amp" need not be expensive. But to go from "good" to "great" can and will be. As you go up in quality, power and/or features, there may be a diminishing "return." I am happy with my cheap, Chinese Ampeg. I have no need at this time of a Glockenlang or Aquilar.
A good amp will have enough power, good short protection, no turn on/off transients, run quietly, have sufficient headroom, and other features will widely vary. Parts will be available when and if repair is needed. Sturdy jacks, good controls, ability to survive rough handling are all parts of a good amp.
In the 60's and early 70's, I struggled with bass amps that didn't have enough power. They were not "bad" amps...just too little power.
During decades at the bench, I didn't have all that many bass amps on the bench. Usually amps that were pushed way too hard (being underpowered) or very abused. If I could get schematics and parts, I considered them good amps. I had some old SS Gibsons, Sunns and others that were problems.