We had a fairly detailed chat about cab emulations here...
I find the JVM line out way too dark. I find it usable with EQ, but essentially unusable without. It wouldn't be terribly hard to mod it to correct that though - the mods I posted before are a nudge in the right direction, but you could go much more dramatic with it by simply tweaking the cap values. I didn't want to go too extreme and opted to play it somewhat safe with those mods.
I like the cab emulation line off the JSX cab though - it's just a solid "generic" 4x12 cab emulation. Add an external EQ and I think the JSX Cab emulation can cover just about any ground you'd want to with a 4x12. And as far as capturing the total response of the amp, it's in the right spot to do that. It's on the speaker line, meaning you get the power amp, output transformer, and how the transformer reacts to the speaker impedance all accounted for in the emulated signal. It's EQ'ed well too, meaning no overbearing frequencies anywhere. Joe obviously either provided direct feedback on the voicing of it, or whoever did knew what they were doing.
One interesting thing to note about the JSX Cab emulation is that it lacks the deep notches and spikes you'd get in the mids and upper mids if you close-miked a real 4x12 with a single mic. The JSX emulation is smoother through the mids and upper mids - more like what you'd get with a multiple-miked cab. Using multiple mics on a cab (and at various distances) smooths out the overall response, and that seems to be what the JSX Cab emulation was going for. That's also simpler to emulate, so practically all analogue cab emulations have smooth overall responses. Those little notches and spikes you do see in the JSX Cab emulation frequency response (on the link I posted) are probably influenced by the amp's output transformer reacting to the changing speaker impedance rather than deliberately emulated by the cab em circuit itself (I hadn't realised that in the other thread).
The impulse modellers are usually based on cab impulses taken with a single mic, so they tend to be much notchier/spikier and "complex". One isn't necessarily better than the other, but they are different. People's impressions of them seem to be based on what they're comparing it to. For the typical project studio person who's used to single, close-miked cabs, they probably find the single-mic impulses more "realistic", but I suspect people used to the big-budget pro studios (like Joe) will tend towards the smoother, multiple-miked type emulation. Of course, you could take impulses in a big studio with multiple mics as well (and they probably do), but that doesn't seem to be the trend I've observed with the impulses I've seen.
I don't know about the JSX head or the stock JSX cab speakers - I bought my JSX cab empty.
As for the build quality, it's not as good as my early-'90s Marshall 1960A cab. There's nothing "wrong" with it, but internally it doesn't have the degree of attention to detail and craftsmanship that went into the Marshall. It's sloppier - tolex cut roughly, screws not angled as consistently as the Marshall, etc.
As for materials, the JSX cab has 1/2" poplar plywood sides, the 1960 has 5/8" birch plywood. So the 1960 is a little thicker and birch is generally the preferred wood. Thicker isn't necessarily better though, because some argue that slightly thinner panels allow the cab to resonate better. If that is indeed an audible difference, then the JSX should have a slightly fuller mid presence... more like the old cabs with more flexible baffle boards. Who knows, Joe might have specified the thinner 1/2" poplar sides for that very reason... or it simply might have been an economic decision by Peavey. Either way, I assume Joe was satisfied with it. When I bought it I was aware of the 1/2" poplar sides and figured if anything it might fill out the mids a little and give a little more "vintage" vibe. I now have it loaded with early-'90s U.K. G12M-25's, and I don't know if the cab is contributing much or not but it does sound incredible (though those speakers also sounded equally good when they were in the 1960A). I believe the baffle and rear boards are birch and MDF, respectively, on both cabs.
Personally, I don't have any problem with the quality of the JSX cab. It sounds fine and I don't doubt that it's roadworthy (though mine sits in a home studio), but it isn't quite up to the standard of my Marshall 1960A in terms of attention to detail on the interior construction. I haven't opened any newer 1960A's, so I don't know if they're still the same or not, but my 1960A is very well built.
_ JVM410H: "Blackface" Clean channel, "Plexi" Crunch channel, "Dual Rec" OD2 channel, 3rd stage bias pot with switchable 100nF bypass cap, switchable 470k/470pF "treble peaker", OD1/2 Orange and Red extra gain pot, signal boost into power amp, PI voltage set to 2203 levels, 5H choke w/ 50uF on screens, -ve feedback pot.
_ Marshall 2203/1959 clone - switchable (with FX loop, resonance control and -ve fb pot) built from a Peavey Windsor 'donor' head
_ Fender Deluxe Reverb clone head
_ ADA MP-1, ART Power Plant, Tech 21 PSA-1
_ 1960A cab w/ Celestion G12-65s and G12-75Ts in X-pattern, Peavey JSX cab with Celestion G12M's (UK 6402 cones)
_ Fender Pro 185 (rebuilt) - open back 2x12" with Jensen C12Ks