Congrats on your MAD amp. For comparison to other similar-tier tube amps, I'd recommend that you go to a local Head-Fi meet. Bring your amp & cans and set up for others to compare, plus you can plug your headphones into other folks amps for a better comparison than you'll get by asking for opinions. Opinions are like... um... fingers! Most people have a bunch of them. The MAD amp has some limitations and benefits compared to the other amps you mention, but you should listen yourself to see how much the amps' profile works with your musical preferences. My opinion is that it does a good job of getting into the music without hyper-detail (some Shiit) overt lushness (some Woo) or blandness (many Chinese tube amps) that I've heard at meets. Depending on the headphone with which it is paired, the MAD can deliver years of great listening pleasure. Mine has, for me. But there's a lot of stuff out there that will leave your jaws hanging in astonishment. All depends on what floats your boat. Your Senn HD650's should be a nice match with your soon-to-be-updated MAD amp. Lloyd is right too with regard to input tubes affecting the presentation. You should build up an inventory of 12ax7 and 5751 types and their variants. Check out this thread for some recommendations. You might want to look into Grados simply because many of them have a truly nice synergy with the MAD amps. I personally have problems with on-ear cans, so not a big fan of the Grado fit, but the sound can be really nice. The last meet I went to my neighbor had a set of older Grado HP-1's (?) with the flat earpads and the sound was really excellent. I spent half the meet listening to that combo with lust in my heart. OTOH, the MAD quickly runs out of gas with demanding loads (Ortho's), so that will limit what you should plug into them. I've liked mine with Senn HD-580's and HD-600's, some Beyerdynamics, and even with Audeze LCD-2's (at low listening levels), not so much with Senn HD-800's or any of the HiFiMan cans. Your ears will be your guide. Have fun! Just remember it's all a means to an end (musical enjoyment) not a scorecard for toy ownership. When you find something you really like... STOP, and enjoy the music, since that's what it's all about. Hope this helps!