well... i own a Mark V and a JVM. i realize that we might not have the same exact taste in tone, but i can't stress enough how much better the Mark V is IMO. don't get me wrong - i love my JVM and have no plans of selling it anytime soon - but if i were in a money crunch the JVM would be long gone before i even thought about getting rid of the mark. if the mark tone isn't for you, then i guess it's just not for you. but i honestly believe that going from mark v to jvm would be a downgrade. i personally havent had any problems getting gobs of sustain from my V. i would agree though that a compressor or light OD pedal might be a good way to put it over the edge, even though i've never felt the need.
it's true that the JVM does have absolutely beyond ridiculous amounts of gain on the OD red modes, pretty much regardless of where the gain knob's set. i normally run the gain at around 11 o'clock on OD2 red (which is still PLENTY saturated) to reduce noise and retain dynamics. due to all this gain, it is also true that the higher gain modes are pretty much unusable without a noise gate once you crank them up.
i had the JVM before i got the V, and i upgraded mostly because i felt the high gain rhythm sounds lacking. OD2 in general sounds way too scooped no matter where the mid knob is set, which translates to sounding very thin even though it has so much gain and bass in its inherent voicing. OD1, on the other and, is great for leads but not scooped enough sounding for high gain rhythm (IMO). if singing leads are what you want though, the JVM will deliver it in spades. comparing the lead tones of the 2 amps, neither is really better for me, theyre just different.
the cleans on the JVM are also great, but not even close to as great as the Mark V cleans. same with the reverb. both sound very 3D on the V, but the marshall's sound kind of flat and just "OK" next to the boogie.
the one thing that bugs me the most about the JVM is how unresponsive the knobs are. no matter where the knobs are set, you just get very slight variations of the same sound; on the Mark V, cranking just one knob (or slider) one way or the other will drastically alter your tone. for example, even with the treble and presence cranked, i just cant dial in a bright enough tone in for my tastes on OD2, and that channel always sounds scooped to my ears even with the mid knob near dimed. it's true that the JVM does give you 12 modes at your feet, but theyre all more or less locked into what theyre "supposed to" sound like, and a couple of them overlap. the 3 channels on the Mark V is enough for me.
since i've gotten my Mark V 4 months ago, it's replaced my JVM as my "go to" amp. when i go to practice/jam/rehearse/write/record/whatever i plug into my Mark V, but i like to keep the JVM around for more options and to have something else to plug into when i want a break from the boogie. after the honeymoon with the V ended, i've started to plug into the JVM more, and every time i do, i plug into the V afterwards and i just feel at home again. all i'll say is that if you are seriously considering the switch, i'd think about it long and hard. it may even end up being better for YOU than the V, and it might not.
it probably seems like i'm badmouthing my own amp a lot but that's only cause i'm comparing it to the Mark V :lol: