I really miss my rack setup. I listen to my old recordings with it and I think "man, I didn't think it at the time but I had some great tone". To tell you the truth, a tube preamp and a tube power amp will give you some amazing and versatile tone for usually less money than a good tube head. I never liked the sound of solid state modelers such as the POD through tube poweramps. The Prophecy II looks good though. Even when I had my ADA MP-1 (very generic tube preamp) into my Mesa 2:90, it was a KILLER set up. I could get it to thump like a 5150, especially when I had the "Deep" mode enabled on the 2:90. Actually before I had the 2:90 I had a basic Peavey Classic 120 tube poweramp and that thing sounded just as good, it just didn't have any presence or resonance controls.
So after going through a few heads in the past year I'm building a new rack, and I started with a Mesa Studio Preamp. If you're looking for a dark heavy high-gain sound like that of a 5150, my ADA MP-1 delivered that nicely. These MP-1's are renowned for their option to have a solid state clean channel too, so you have a tube drive, tube clean, and solid state clean all in one preamp. There's an assload of cheap discontinued tube preamps by Rocktron and Digitech that will give you a good sound. If you have the money, a used Mesa Formula or V-Twin might make you very happy as well.
As I wasn't in a band where I had to compete with a 400 watt bass amp and another 100 watt tube amp, the 2:90 was wattage overkill for me. The Deep and Half-Power modes were a great feature, but you'd be plenty happy with a Mesa 50/50 or 2:50.
So if a Prophesy runs you $600 and a poweramp runs you $600, you've got a $1200 amp that's 10 times as versatile as most heads in the $3000-4000 range.
Racks rule.
