I think tubes/valves are definitely the way to go for guitar - with the possible exception of the old Acoustic 150 head or the Roland Jazz Chorus combo.
For bass I think it's less a matter of being definitely preferable and more a matter of ones personal preference. There are some very good sounding SS bass amps. I can't really say I feel the way about guitar amps.
Speaking purely for myself, I prefer seeing and hearing some "glass" in my bass head. That's what I cut my teeth on and grew up with, so that's the sound that always seems most right to my ears. But since it's not always practical in terms of weight and money when it comes to stage volume levels, a good SS amp also works for me if I need major wattage.
For bass I think it's less a matter of being definitely preferable and more a matter of ones personal preference. There are some very good sounding SS bass amps. I can't really say I feel the way about guitar amps.
Speaking purely for myself, I prefer seeing and hearing some "glass" in my bass head. That's what I cut my teeth on and grew up with, so that's the sound that always seems most right to my ears. But since it's not always practical in terms of weight and money when it comes to stage volume levels, a good SS amp also works for me if I need major wattage.