Tubes are a practically "ancient" technology that pre-dates solid state integrated circuits or op-amps (but that in no way implies they are inferior because they are older). Tubes are literally little glass domed objects that plug into the circuits of tube gear. There are a million different kinds, shapes and sizes of tubes. Tubes ultimately wear out and have to be replaced. This can get expensive if you want the good old tubes which are disappearing rapidly.
Op-amps are tiny little solid-state wafer-like devices with prongs that attach to the circuit board. Until recently, most op-amps sucked, the chief benefit being their relative cheapness, and compact size, but they've come a long way since and can sound dazzling in the right circuit.
You cannot substitute one for another-- an amp is either designed for tubes, or for op-amps.
It's impossible to make generalizations about tubes vs. solid state. Each amp, each circuit will sound different, whether it has tubes or op-amps being just one factor in determining the sound.
That said, tube-o-philes generally like tubes for their delicious euphonic sound, it's very musical and seductively intimate, but in some cases perhaps not strictly "neutral". OTOH, I don't think it can be successfully argued that solid state devices are inherently "more neutral"; they can also have a signature sound of their own, which can be more speedy, more impactful, edgy, and possibly more detailed.
Again, these are very broad generalizations that shouldn't really even be made.
Maybe others can elaborate. Gotta go. Bye.