I've directly played a 2005 KT-88 Pro Mod aside from my own 2011 Super Stock, and neither amp has issues with harmonics, dynamics, weird mids, or quackiness, but they are different to QRs in their own separate ways:
The Pro Mod is a 3 preamp tube model, so the cleans are basically non-existent, I would go so far as to say that it's a waste of a footswitch button and front panel space, and the gear 1 classic rock aimed tones can be a bit sterile for that application but can still pull some great tones, (this one has gears unlike the 2005 mentioned earlier in the thread), but gear 2 and 3 are fantastic for metal and hard rock. It definitely has a different vibe than the Quick Rods from that era that I've heard recordings of - more of a modern, compressed voicing before the Nitro came on the scene.
The Super Stock has literally got the best Marshall-like clean channel I've ever heard, even heading into good Fender/Mesa territory. It's how you imagine a plexi should be, but more full bodied if you want it and none of that harsh bit of sizzle/grit once you push a bit of power through, and the OD channel is basically a wound out, channel-patched plexi on steroids, (with the sizzle, but not the grittiness that a lot of plexis have). You won't get modern metal tones out of it in a million years but it'll go into thrash territory dimed out on OD2 with a bit of boost. These were made with a 4 tube preamp from around 2007/8 until basically mine from what I recall, (I ordered mine after they were removed from the store in early 2011), and were meant to be a pedal friendly alternative to the QR/PM which were notoriously finnicky with boost/ODs/dist pedals in their original 3 PT form; but if mine is any indication they're far more than that, they're probably among the earliest "plexi with modern convenience" amps ever made and I'll honestly take it to my grave. I've only ever seen two for sale second hand in the last 15 years though, if mine
was the last one it's number 97.
I do fully agree about them cutting through very effectively and have no doubt this one would too, but that's not the same thing as a good quality tone. For example, the harpsichord is one of my favorite sounding instruments but tends to get lost or hard to hear in ensembles. Does this mean it's lacking in quality of tone? Hell no, it just by nature has trouble cutting through and same IME with mark amps and especially the mark iii (more scooped IME), but in what I at least interpret as quality in tone and feel, I find it a superior amp that unfortunately struggles by nature to cut through and especially compete with more Marshall-y amps. For me the best answer is an amp that has both quality tone and cuts well like a good vintage Marshall either stock or with a good mod (Naylor, SLO or Steavens too). Just my opinion I guess. Maybe I'm missing something, but I'm just not hearing what Splawn's bring to the table that's unique in a desirable way that I can't get with other amps I've kept and believe me I'm trying to look for it as I do when getting new gear that I explore. I do really like the core Marshall sound though that is getting blocked by the overlying Splawn flavor from the mod. I guess I made a NAD thread sort within this thread LOL
You're right, they're not the same; good quality tone is subjective, cut through is not. Good quality tone also has a severe case of diminishing returns in a live setting and in terms of dollars spent on a head. It's absolutely the case that a Mark III is an inferior choice to most Splawns in that use case, regardless of personal tone niggles - making your whole band sound like mush is bad juju and amps are just a tool at a gig, no one even looks at the back line.
I'll caveat that by saying that you are probably right in a studio or home setting, I've never even seen a Nitro, and I don't know about Quick Rods specifically, because I've only ever seen a 2014 model used live in a smaller hard rock act that was touring here, and never played through one and I really like Mark IIIs, they're just kinda useless to me in reality. But that dude's QR's tone and clarity ate easily 95%+ of live tones I've ever heard, played through a bog standard 1960A cab - not exactly a rig in the price range of Bogner/Wizard/Soldano, (and being in Australia, Mesa pricing is
insane), nor with a lot of the potential issues that come with touring a vintage JMP or 2203.
I don't think it's really about what they bring that's unique, the QR specifically is meant to be an amp that can do the brown sound reasonably easily/reliably at its core. The real question is why do the small group of people that actively hate on Splawn on the forums/socials and in youtube comments always suggest that the person asking for info double or triple their budget? It's no wonder that so many people just go buy a Helix.