Heat really isn't a thing with tube preamps. They make a little heat but its not significant- if that is really a problem you have much bigger fish to fry! Reliability is another thing, but IME you can leave a tube preamp on for some pretty long times (days) and not have any issues. They are not the same as tube power amps in this regard. Our preamps are warmed up very nicely with only an hour of operation so I see no need to leave them on all the time. OTOH many solid state preamps need to be left on continuously as many of them take about a week to warm up.
The problem IMO is there simply aren't any solid state preamps that have the relaxed quality of a good tube preamp. Every one I've heard errs a bit on the sterile side. But if you want some tips, I would be looking for one of two kinds of solid state preamps.
The first is one based entirely on high performance opamps. The reason for this is something called 'Gain Bandwidth Product'. High performance opamps can have a high enough value of that so that the feedback they run is actually capable of also suppressing the distortion caused by the application of their feedback (I know that sounds a bit recursive but that's the way it is). Its a simple fact that in most solid state designs the application of feedback (usually a requirement for them to be linear) causes them to be brighter and harsher than real life because the feedback used is an insufficient qauntity- and therefore unable to correct the distortion it introduces by its application! If the circuit runs enough feedback it can then be neutral and musical- without added brightness and harshness (and so sounding like tubes).
The other kind of preamp I would look at would be one that is entirely single-ended, so it can make a 2nd harmonic, and then be otherwise a very simple zero feedback design (probably meaning its MOSFET based) with perhaps only a single transistor and not much else in the line section. This will be a bit less neutral but still have some tube-like qualities owing to the 2nd harmonic and no feedback.
The problem IMO is there simply aren't any solid state preamps that have the relaxed quality of a good tube preamp. Every one I've heard errs a bit on the sterile side. But if you want some tips, I would be looking for one of two kinds of solid state preamps.
The first is one based entirely on high performance opamps. The reason for this is something called 'Gain Bandwidth Product'. High performance opamps can have a high enough value of that so that the feedback they run is actually capable of also suppressing the distortion caused by the application of their feedback (I know that sounds a bit recursive but that's the way it is). Its a simple fact that in most solid state designs the application of feedback (usually a requirement for them to be linear) causes them to be brighter and harsher than real life because the feedback used is an insufficient qauntity- and therefore unable to correct the distortion it introduces by its application! If the circuit runs enough feedback it can then be neutral and musical- without added brightness and harshness (and so sounding like tubes).
The other kind of preamp I would look at would be one that is entirely single-ended, so it can make a 2nd harmonic, and then be otherwise a very simple zero feedback design (probably meaning its MOSFET based) with perhaps only a single transistor and not much else in the line section. This will be a bit less neutral but still have some tube-like qualities owing to the 2nd harmonic and no feedback.