The ESP Guitar Company

if your preamp has tubes then ime don't shortchange it with a solid state power amp; get a tube power amp, but for some brands they're expensive; still for other brands and used they're not.

some tuberzez:
peavey classic 50/50 and some other wattage models in this series
carvin TS100 or T100S
mesa 20/20, 50/50, and a bigger one
marshall 20/20, and one or two bigger ones
vht's

i've owned the carvin, it was okay and had nifty features but it didn't wow me, but still way better than SS poweramp.
i have the mesa 20/20 and used to have the 50/50, both are awesome!!

decide of tube type matters, the smaller ones usually use EL84's which to me sound fairly similar to EL34's;
bigger ones may use EL34's, 6L6's, or KT88's,
imo KT88's are my fav, EL34's sound similar to them,
and 6L6's sound different,

so IMHO el84, el34, and kt88 often have similar tone but not exactly the same;
and 6L6's are noticably different to me;

the 6505/5150 comes stock with 6L6 fwiw.

what is the preamp you'll be using?

I've heard that VHT's are killer...

The 50/50s and up will be closest to a normal tube head if that's what you're after, just bear in mind they're usually stereo poweramps, meaning you have the rated wattage per side, and thus will need two cabinets or a stereo cabinet. And it'll be loud - a 50/50 poweramp will be like having 2 50watt heads on at the same time.

Personally I'd go for a 20/20, but that's because I love EL84s.

As Gaijin said, EL84's are close to EL34's, but also vastly different IMO. EL84s are a softer distortion, more laden with those upper harmonics due to their lower power handling. The sound of EL84s being driven to the max (which isn't as loud as EL34's being driven to the max!) is the main reason I love playing guitar.

that randall mts system has always been very appealing to me, it has cost so much new tho i've never taken the leap, but i still plan to try it out someday if i ever can.
i had wonderful results using a 12ax7 equipped johnson millennium head played thru the mesa 5050 power amp, it was so damn good,

the johnsons tho had software failures, i sold mine while it was still working fine but i did not trust it, i owned another one that crapped out and i had to pay heavy shpg cost to get it to johnson for repair; when johnson ceased to exist i sold both amps, knowing i couldn't get them repaired anymore.

but that is a lot of reason why i think the mts system probably kicks ass; i don't think it has any digital/software afaik, i think the mts is a killer concept.

using the line 6 will work.

you want a sweet rack metal amp setup?
power amp - mesa 2:90(make sure you cabs match the amps impedance)
pre amp - mesa triaxis(not that you'll need 128 channels, but it's nice to have)
fx - tc electronics gmajor or the like(an Eventide would be most sweet to be honest)

As I am totally unaware of your budget I will go full blown, and that is what I'd get. Toss in a couple 4x12 recto or road king cabs & I'd be happy like a kid with a full box of super sweet golden grahams & gal of milk & a huge bowl/spoon. :D

However, since you say you like the peavey 5150 I recommend the Rockmaster Preamp-all tube & perhaps a 50/50 power amp, also by peavey.