Hooked up the interface box to a couple speaker amps. The difference between them and the Jotunheim R are night and day. Granted, all I used was an old Sherwood amp from the '90s and a Fosi Audio chi-fi desktop amp.
The Sherwood is 100 Wpc into 8 ohms. It sounded muddy and undetailed. Bass was elevated and bleeding into the mids. The highs were muffled and very veiled. Overall the sound was just an awful, bassy, smeared mess. Listened to this for about 20 seconds and moved on.
The Fosi Audio amp is 50 Wpc into 4 Ohms. It was much clearer and more detailed than the old Sherwood. The bass was still elevated compared to the Jot, but the mids and highs were much cleaner. Still sounded a bit veiled. The soundstage was smaller, but the imaging was just as good as the Jot.
The Jotunheim R (baffle compensation off) has tight bass with no bloat, crystal clear mids and highs, and is speedy and very detailed without being as harsh as something like an HD800S.
I think it’s safe to say that there is some kind of EQ for the bass coming from the interface box. The Jot R has the baffle compensation toggle designed for the SR1a. It’s basically a shelf EQ that lowers everything over 500 Hz so the bass is more accentuated. Using this feature doesn’t sound terrible on the GL1200 if you want elevated bass, since headphone frequency response is so flat. The bass elevation coming from the interface box is more centered around the sub bass region I think, but it’s hard to tell exactly where it starts.
All in all, these headphones are extremely amp picky. The sound will change drastically depending on what you are using to power them. I only wish I had a speaker amp that I knew was very analytical and detailed, so I could see if the interface box itself is causing the loss in detail compared to the Jot.
@heavyharmonies I was at 50-60% volume on the Fosi Amp for a decent listening level on most tracks. I went up to 80% for my quietest music. So I think the least you can get away with is 50 W into 4 Ohms, but if you want to mess with EQ, you should go for 100 W.