A copy/paste from another forum, but I thought it might be nice to share my 2c with the guys here :) I've only had it for a week, so anyone with more experience who has something to add or disagree with, please feel free!
This is all based on using a V30 loaded Carvin Legacy 2x12", and my LP Custom and Ash custom into the high sensitivity input.
AFD mode:
This is meant to be based on a 1959T with the unused valve for the tremolo circuit modded to become an additional gain stage.
It's there, "that sound". I was sceptical at first, and I realise that there's a whole lot more to a recorded sound than just the amp, but this mode totally sounds, and more importantly, reacts the way you'd imagine it would, based on the guitar tones on the AFD record. It's a fairly thick, meaty overdrive, quite girthy and focussed in the lower mids rather than fat and open in the traditional Marshall way. There's a fair whack of treble but this is tempered and balanced by the rest of the sound. One of my favourite things about it is the way the low end reacts to palm muting, not just for power chords but single notes as well - it's tight and yet "springy" or "bouncy" (ooh-er), which gives the notes this great percussive feel.
There's quite a lot of gain to be had here (5 preamp tubes' worth!), and I found that most of the special tone in this mode comes from preamp drive, so you can dial in the master fairly low and still achieve that sound fairly easily. Diming the gain gives you heaps of it, but it doesn't really sound "br00talz" but it can get a little stodgy if you don't adjust the EQ to taste. The EQ works pretty well, much more reactive than my Vintage Modern (but that's not saying a lot, the VM is mainly controlled through it's high/low preamp controls), you can scoop it and stuff but to be honest I haven't really experimented much with this, cos it sounds so good set around 7 for everything! It'll definitely do most 80s/90s metal fairly easily.
Clean sounds - none to speak of with humbuckers, but I managed to get a reasonably decent rolled back clean using the split settings on my Ash custom, and that was good enough for me.
#34 Mode:
This mode is meant to be based on a JCM800 modded for Slash by Frank Levi ex-S.I.R. As Ryan says above it seems to be basically a JCM800 with a HUGE amount of treble. Supposedly, this is exactly the same as the modded 800 Slash has been using since the Use Your Illusion days.
A lot of people have been ragging on this mode, I think mainly because everyone sets their EQ for the AFD mode, and switches over expecting magic, but then it sounds thin and reedy. This really kinda sounds like a completely different amp, and needs to be tweaked to suit. This is where the EPA (fancy name for the attenuator) comes in. Dialing down the power lets you push the poweramp to get power tube overdrive into the mix to give the sound some meat without deafening yourself, your neighbor's dog and small rodents within a 2km radius. It's still REALLY trebly, even with presence and treble turned down, but I can imagine that this will cut through a live mix like a crazy Japanese knife.
The perfect tonal baseline for this mode is the song Back from Cali on the new Slash album. It sounds pretty much EXACTLY like that. Down to the volume-rolled-back intro.
There's not really as much gain in this mode since it doesn't use the extra gain stage. All I can say is, if this is really what Slash has been using for the past 20 years, I've gained a LOT more respect for the fluidity of his legato licks. Due to the lower gain, this channel cleans up a lot more readily with the guitar's volume knob.
Versus the Vintage Modern:
Totally different animal IMO. The VM is much more open, and has a midsy KT66 roar, while the AFD is more like a velvet-gloved punch in the face. You can get close to the #34 mode with the VM but I think the gain structure is just completely different. I'm glad too, cos I would have been gutted to have got the AFD and have the VM sound like it
The VM remains firmly in the hotrodded 60s/70s hard rock camp for me, which is a really cool place for it to be. I have to sell my VM due to buying the AFD but I can see myself owning another one in future.
Electronic Power Attenuation:
The power scaling is great, and as I mentioned earlier, is pretty much integral to getting the #34 mode to sound good at sensible volumes. I can't really hear any significant degradation in tone till you get close to whisper volumes, and seriously, why would anyone want to play this amp that softly? I can see this being REALLY handy at gigs, to be able to dial the tone in and then just take the power down a notch. The power scaling is not continuous - it happens in a stepped fashion, but there are enough steps that it doesn't matter.
Verdict:
Very cool! It's not the be all and end all, but neither is it completely a one trick pony. There are at least two very good tricks in there and more to be found I reckon. To an extent, the AFD mode is always going to be used for that awesome overdrive sound, but the modded 800 mode definitely has a lot of potential for sonic variety in there.
Power scaling is a god-send, and I wonder if Marshall will roll this out for non-LE/signature stuff any time soon. Quite a game-changer if they do.
The amp looks like a crazy mongrel aesthetically (representing all the various influences on it), and I think it's cool since the Slash signature aspect was not overdone in any gaudy fashion.
I'm glad I got it, now for the YJM100... Actually, if I had to chose one, I'd definitely chose this over the YJM. The YJM to me is basically a Plexi with power scaling, the other features are just built in stompboxes. At least this is something really quite unique. And of course I am a huge Slash fan! Strangely enough, this is the first Slash signature kit I've owned. To each their own of course, I know heaps of guys here are hanging out for the YJM, and I'm sure it'll be really cool.