I am in exactly such a situation at the moment as my new Ampeg PF-T50 just arrived. I was in a rehearsal room today to compare it with some Class D amps and Solid State with a lot of cabs, since I was worried about Ampeg PF-T50 volume.
Some of the impressions (speaking from my point of view and my experience from loud rock bands and big stages) are:
-Ampeg Pf-500 Class D (300W/8Ω) is is twice as loud as tube Ampeg Pf-T50 (tube 50W) - (plugged in the same Ampeg PF-410 HLF 8Ω).
-Ampeg Pf-T50 (tube 50W) + Ampeg PF-410 HLF 8Ω is about as loud (maybe 10% louder) as Gallien Krueger Fusion 112 Combo (400W/1x12'' combo).
-Gallien Krueger Fusion 112 Combo (400W/1x12'' combo) is about 30% louder than Ampeg Pf-T50 (tube 50W) + Orange OBC112 (400W/8Ω).
-Gallien Krueger Fusion 112 Combo + Ampeg PF-410 HLF (800W together) is louder than Gallien Krueger Fusion 112 Combo + Orange OBC112 (800W together).
-Gallien Krueger Fusion 112 Combo + Harely Benton 210 cab is louder then Ampeg Pf-T50 + Orange OBC112 + Harely Benton 210.
-Gallien Krueger Fusion 112 Combo + Ampeg PF-410 HLF (800W together) is as loud as Gallien Krueger RB1001-II Solid State + Ampeg PF-410 HLF.
We tried many more combinations (with some more Class D heads, cabs and combos), but the conclusion IMHO regarding power and volume is as follows:
Tube 50W is about as loud as Class D 250-300W. Both are loud enough for most of the small clubs and venues, but it is very likely that in a loud band with loud guitarists it may not be enough (indoors, without FOH)
Class D 400W in a single combo is very similar to the one previously mentioned.
However, Class D 500W is loud enough for almost anything (Fende Rumble 500, GK MB212-II...indoors, without FOH)
Class D 400W combo + ext cab (800W together) or any Class D 800W head + at least two speakers is louder than You'll ever need (indoors or outdors).
Tube 100W + 1x410 or more, is probably louder than You'll ever need (indoors or outdors).
Solid State 750W + 1x410 or more, is also louder than You'll ever need (indoors or outdors).
I apologize for this long text, but I had to write it because I am currently under impressions