Okay, had a chance to plug into the Gustard, and here's my take on a few samples I know very well:
- the Gustard with the 650's is
smoooooooth. In a good way and a not-so-good way depending on your tastes.
- Listening to Adele and Natalie Marchant, the dusky female vocals come through beautifully (this is the gorgeous mids of the 650's so no surprise). The Adele was from a 24/96 vinyl rip which I know has some wow-and-flutter and a few pesky pops - none of that was the least evident.
- Moving on to something edgier: Dave Matthews and Tim Reynolds - Live at Radio City. A very good acoustic recording where the strings are very raw and edgy, very clear. Just enough bite in the strings to feel the quality of the recording - lovely sound but missing a bit of the bite this recording has loads of.
- Guns 'n Roses - Appetite for Destruction: just not enough edge. Turning it up to try test the waters and the mid-bass bloom started to take over.
- Vivaldi - The Four Seasons - Mariko Sen Ju - beautiful violin, very good ambient. Soundstage was about as good as I've heard the 650's (which are quite intimate to begin with) once the orchestra kicks in. Not the 650's favourite genre (at least the full-on orchestral passages) but good timbre on the strings and a nice warmth
- Yello - One Second - a little too polite once again - needs a bit more bite and edge.
How does it compare to the modded Crack? It doesn't really. From reading above you'll hopefully get the impression it was clean, smooth, warm, and oh-so-polite, but lacking a bit in detail and attack. In that way it is very much like the 650 itself. It sounds almost more tubey than the Crack with some warm Mullards. If I heard this from my Crack I would be happy for some music, but definitely rolling in brighter tubes for anything rockier, or faster, harder music. The Crack just seemed to swing the sharper transients better, whereas the Gustard smoothed them down, Kinda like the Appalachian mountains versus the Rockies - do you want to relax and enjoy the scenery or jump out of a helicopter and ski for your life.
With the Crack you have options to tweak, not so much the Gustard. You can roll op-amps, but I doubt with the same range of effect. The magic the Gustard brought to my LCD's just didn't rub off on the 650's. It's good, but not Crack-good.
I think this rambling has just confirmed something that's becoming a truism: for the 650's get an OTL tube amp with a high voltage swing (Crack, Ember, Woo, etc). For the planar world get something with large current flow like the Gustard, Cavelli, Lyr, Audio GD amps.
If you have to choose one you need to ask yourself: do I want only the 650's, DT880 premiums and other high-impedance cans (get OTL) or do I want a range of cans including lower-impedance cans, in which case the Gustard is great bang-for-the-buck.
Hope that helps, even if it's maybe not what you wanted to hear.