We need some pretty hefty power, 50 to 100 watts or a loud 30.
Why do you need that? Where are they playing? What kind of music is the "old school" music?
30 watts is a very loud amp. 50 watts is waaay loud. 100 watts is deafing, IMO & more appropriate for an outdoor stadium.
I was using a 56T with 6BM8's (maybe 9watts) mic'd thru the PA and was asked to turn it down in a band with keyboards, loud drummer and 3 singers.
I played a Music Man 50watt and later a Carvin 30watt for a few yrs & I am confidant some of my hearing loss is related to playing those two amps. Sad part of it is, that I never could find my "tone" with those amps. I regret playing amps that loud. I can remember my ears ringing after playing.
I see some younger players who have had SS amps not realize how loud tube amps are.
A good lower watt tube amp that has some clarity to it will cut thru a band mix quite well. A Deluxe Reverb is 22 watts and is considered a fabulous amp for small clubs. I have two amps now. One is about 14 and the other is maybe 23 watts. Both amps have a very clear sounding overdrive that isn't muddy at all, IMO.
My opinion is that some of the production amps like HotRod, Peavy, Crate ......etc...... don't have much of a clear tone and that the overdrive is somewhat muddy sounding. And invariably, I then see someone turn up the volume trying to hear their muddy tone amp better only to have louder mud. The band tone then sounds like mush to me.
In contrast, a lower watt amp that has great tone and clarity can cut thru the mix just fine. Numerous forum members have commented how their lower watt amp cut thru better than someone's HotRod for example. I think Dynaflow has commented on this for example several times?
Building something like a 5E3 Deluxe (maybe with 6L6's) or an 18watt might be an excellent choice. You can look to even convert an old Bogen, Hammond, etc..... PA and put it in a cabinet with a nice speaker. I've done that for under $250 several times. Ebay occasionally has converted PA amps into guitar amps for reasonable prices.
That's my 2 cents worth. With respect, Tubenit