Just looking for general comparisons/opinions on the following:

Marshall DSL 50w
Marshall JCM 900 100w
Mesa Boogie Dual Rectifier
Peavey 6505
Peavey XXX
Peavey JSX
Splawn Quickrod (before gears)
VHT Pitbull 50w

Where do they shine? What do they lack? Yadda....

I've tried most all of them, and like them all, for different reasons, maybe someone could bring something up I had not thought of?
I Have a JCM 900 SLX 50w. tone on it is really nice and smooth. its not a clean amp nor is it incredibly gainy without pedals (can barely get heavier than Maiden running on full gain ie turned to 20). I love it suits my style perfectly, cab also plays a really big part tbh my head sounds shite through a peavey valveking cab :P, but amazing through my 1970's plexi cab with greenbacks. DSL's are too fizzy for my liking when cranked never much cared for them. In short 900 isnt very varied but its fantastic at what it does, which is hard rock n roll ala Zeppelin, AC/DC and surprisingly sounds very hendrixy with single coils and turned up loud with the gain down. copes with effects quite nicely too in the main signal chain (dont use effects loop). one thing i will say is DI out on it is pretty dire on mine so i just mic up the cab instead.

tl;dr JCM900 Good amp in narrow field of hard rock
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I used to own a JSX and I now own Mesa Rect-O-Verb single rectifier.

Personally, I hated the JSX. I've heard plenty of good things about them, but I didn't like mine. I'd thought about adjusting tube bias and upgrading from my Peavey cab to see if these might be factors working against me, but I instead got the Mesa.

To me, the JSX is a little less open sounding (some may describe this as more "warm") and the distortion has a cleaner rock character. It's clean channel is really good and has a lot of headroom.

The Mesa roars. The distortion is more guttural and thick, yet the modern channel gives it a good openness and clarity. I run a tubescreamer clone in front of mine so I don't have to run the amp's gain so high. Doing this seems to reduce fizz and tighten the sound up.

The Mesa, to me, does metal... does hard rock... does decent clean (I'm still working on getting the cleans right). The JSX didn't do much for me... but then again, some people will play no other amp.

If you play metal, or heavier rock, and like a really aggressive, guttural distortion, go Mesa. If you like a distortion that's more clean and crisp (maybe sings more than it does growl), then you might look elsewhere. Don't get me wrong, Mesa's do more than metal, but... honestly, it really comes down to what your ears tell you.

May i suggest...Randall MTS series? one of the most versatile (while staying priced relatively reasonable) tube amps you'll run across...being able to switch preamp circuits that model anything from a fender to a marshall to a mesa to all the other big names, as well as switching power tubes from EL34 to 6L6....well that just makes for an ultimate do-all-styles-better-than-good amp period. I love my RM50 combo, it's practically a marshall in a Randall disguise lol...but seriously...these amps are so beautiful. They've finally started picking up recognition here in UG over the past year. I remember when i first got mine two years ago, only a few here on UG owned them...now it seems as though the ratio has gone up, which is always good .
My MAIN Gear
"They call him the 'Sand Spider.' -Why? -Probably because it sounds scary"
*Agile AL3000 Les Paul w/ Alnico IIs
*Randall RM50
*Dunlop CFH
*fellow LEO feel free to give a shout out
I violently dislike all the peaveys there. IDK why, they just never shone to me. VHT and Splawn I've never tried, but both carry a pretty hefty reputation. The Marshalls & Mesa I can help with though.
Dual Recto: sounds brilliant for pretty much anything, but needs a LOT of tweaking.
DSL 50 - nice amp, good at everything but brilliant at nothing
JCM 900 - I do like JCM900s, but honestly, I'd just get an 800 and boost for (IMO) more flexibility and pretty much the same tone.