Yep, you got the basic idea of them correct. I can't comment on the Mutron, other than having had a Qtron, and that the Mut/Q are the juiciest and funkiest around, but not necessarily the best for stacking with other effects because it so resonant and can be overpowering.
Grinder:
*Full featured Meatball clone with an extra 'Frequency center' notch (4 settings instead of 3), and 'upgrades' (i suppose some or any-at-all could be subjective) like high quality buffers in the effects-loop, exntended EG range for longer/wider sweeps, anti-pop something-or-other for the footswitch, etc
*Super fat, can sound great without volume spikes, and with resonance dimed it gets pretty wet and spikes a bit, though not as wet or spiky as a Qtron or Mutron. With the resonance lower, you can get some super fat and whompy bass, very similar to the Fatman to my ears, partly by tone and partly by feel; the excellent envelope control contributes to this.
*Exp inputs for wet/dry blend, depth and decay. With an exp pedal plugged into the Decay input, and depth/attack/decay all full clockwise, you can sweep the filter manually. The depth knob will control how much available sweep you have when rocking the pedal; Rolling back the Decay from this position can unlock some killer tones-- imagine quickly sweeping the pedal forward and opening the filter and it follows your foot speed, and then quickly close it and the filter will slowly close based on the decay setting, and it feels like a slow envelope.. drifting down on a cloud.. haha. I just plugged in to make sure I had the correct input, and played with it for a bit.. good stuff.
The thing with the Grinder in my application is that is I don't need a funky filter, so it's become a jack of all trades, master of none. I can get the clean and super fat whompy bass with the Fatman, and for the few funky tunes I can easily get away with one of my other filters... so i struggle with keeping it and taking up the space, or pulling it to make room for something else. Then do I keep it 'for recording'... or sell it to fund other gear? I dunno.. Great filter though, excellent build quality and sound- if I could only have one funky filter, this would probably be it.
Photron
*Similar functionality layout to the Grinder, with an added LFO section.
*Tonally it is comby like the Moog, but with a much more focused resonant range, and much brighter. It has amazing clarity and when engaged but fully open, it does not color the signal at all... very clean in that regard. This tight resonant band allows for more precise, or tighter and more resonant modulation without feeding back.. Like drawing with a ball point pen VS a sharpy; or better yet, like trying to play chords with out getting muddy (or on the flipside, shredding) with thick bass strings VS thin guitar strings.. if that makes any sense at all!
*As an envelope filter: its not as fat as the Grinder and its envelope and corresponding knobs are nowhere near as intuitive. The character is somewhere between the FX25/Prometheus and a Qtron-- it can be set to get whispy at the top of the sweep and can be super snappy with a big bottom-out like the FX25 and to a lesser extent the Prometheus; though it can get very wet and resonant like the Qtron, it doesn't have the wide resonant range so it sounds tamer, and I find it hard to get a fat envelope in the low notes much more so than high notes... as an envelope filter, I bet it kills on guitar.. but for bass, I'd look elsewhere if you're just looking the the end-all envelope filter. For synth stuff its great, has a nice character and is unique sounding enough to compliment the Moog nicely, especially since it has BP/HP as well. If I could only have one filter on my synth board, it would probably be the Photron.
MoogLPF
*Fatness all around. Darker than the others and very coloring, for better or worse (i like it). The fattest for dub and synth bass in general, capable of many tones and easy to adjust between settings with its knob/switch layout. Sounds great as an envelope filter, but I think there are others that sound as good, and may not be as versatile, but are quieter, better bypass, smaller, etc. For synth stuff they come highly recommended for good reason.
lol
and I'm afraid if they have one, it'll probably have to come home with me. 
lol
When I've ever seen them (and based on reputation), they go for around $500-600 when they show up. Course... I've never played one, so maybe that IS a bargain! lol
But I certainly understand, there's just something about having an original. 
) Here's the current list:
