If it was a touring amp or even slightly looked after then the 5881's probably got swapped with like for like.
Soldano use those valves because they are tough. Likewise with their choice to go with JJ ECC83S'. Those are a tough preamp valve, mechanically sturdy and not prone to going harmonic. Some people might think that extra reliability is worth the slight trade off in tone.
I like to use JJ ECC83S with a Tungsram 12AX7 in V1, which to me makes the amp a bit more aggressive. My SLO currently has TungSol 7581's in the power section, but I might try KT66's next time. I've used JJ 6L6GC's with good results in a bunch of amps but they recently seem to be less reliable. I don't mind the stock 5881WGC's though.
It's hard to know what to recommend. I usually just swap thing around every so often and stick with what I like. I don't think of it in terms of "This sounds more richly 3D and harmonic in the low organic overtonal touch sensitive area extended bandwidth"... If something sounds better I just go with it. I might change my mind a few months down the line. I chose the Tungsram over a Mullard, Brimar, RFT, Philips 5751, Sovtek 5751, JJ ECC83S, JJ ECC803S, some EHX valves, and some long plate Sovteks, and a Ruby 12AX7. You don't have to go NOS to find nice sounding valves in my opinion.