I kinda get the difference between them in guitar amps, but which one is better in bass amps?
Briefly transistor amps are much cheaper so you can buy more watts for your money but when they overload they sound awful so you need loads of power. tube amps sound actually ok when they overload which is why guitarists like them but bassists don't generally go for an overdriven sound. Transistor amps have circuit designs that reduce distortion to tiny levels and are thought to be clean or clinical depending on your taste. tube amps often have higher levels of harmonic distortion and restricted frequency response and are described as warm by those that love them.

In practice through the sorts of speakers we use for bass you shouldn't be able to hear the difference between well designed amps of either ilk so long as you don't get near the maximum levels and the differences you hear are often the differences between manufacturers and different models rather than due to tubes or not.

Tubes are less reliable than solid state as the tubes burn out after a year or two (depending on how they are treated) budget £100 a year for replacement tubes.

Having said that a band I heard last week sounded great with an Orange valve head and 4x10
I own one...love it...and yes, i describe it as warm. A tube amp usually doesn't sound as tight as a transistor(SS) amp. They can get pretty tight but would say not as tight as a SS amp because of how a tube works. I've had my tube amp for almost 2 years now and am just now thinking of replacing the power tubes because I'm getting less clean headroom than before, that means it distorts at a lower volume than it used to. Preamp tubes can last for many many years though. Especially in a bass amp since you usually don't overdrive them like you might on a guitar amp. I'd say average life for a preamp tube in a bass amp might be between 3-6 years from what i've heard but I've never had one go out on me. I blew a fuse and it took out two preamp tubes with it but that was because of a power surge so I can't blame that on the tubes.
dean edge one 5 string
Schecter studio-4
Samick fairlane-6
Ibanez sb900
Ibanez btb775
Fender p bass special deluxe

Dean Del Sol
Ibanez prestige rg2610

Peavey TKO 65
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Quote by the_perdestrian
listen to revelation, for he is wise in the way of bass-fu
Two things that immediately hit you with tube/valve amps are weight and price.
Tube amps are far heavier than SS amps due to the massive transformers that have to be used although developement with the use of torroidal transformers will see some reduction in weight.
Tube/valve amps are more expensive to produce than SS amps.
An old Orange 120 watt will truly more than compete with 300 watt SS amp; I know I recently compared.
Now that I've got used to my Ashdown Little Giant 1000 @ 7.7Lbs/3.5 kgs there is no way that at my age I want to be lugging 20 kgs of transformers and milk bottles around at gigs.
G&L L2500
Squier Affinity Jazz Bass 5
Ashdown RPM pre-amp
Ashdown Little Giant 1000
300 watt 15" powered cab
450 watt 15" powered sub bass cab
2x10 + horn
1x15x10 + horn