HD650 user here

The HD series are pretty amp dependant actually. Certain amps like La Figaro is able to bring out the sweet vocal out from Hd600. But it's never sibilant for HD600. It doesn't have that spike like his bro Hd800 do. So it won't be fatiguing after long hours of listening. But I only compared it in certain amps in Starpicker. Still ended up with Hd650 in the end for a better all rounder and a more body. But Starpicker have a lots of amps that pairs really well with sennheiser HD6xx series. I love the pairing of La Figaro and Audio GD. But now looking to try out little dot mkiv.

Oh. That's a decent source. But they goes well with tubes also. It's hard to justify the Rm700 difference. As for me, both of these headphones are on the same league. For Hd650, it excels in the bass department when compared to Hd600. Hd600 have a more forward mids compared to Hd650. It's back to preference. Or u can invest in a Hd600 and the Rm700+few hundreds more bucks for a tube amp to go with it.

What a coincidence. I am using Aune T1 too for my Hd650. Hvnt been able to fund an upgrade of amp lol. Both the headphones are at the same league as I mentioned. But upgrade cable does a great magic on my HD650 too. The Symphonic Cable. You should really try it. And also, a headphone calibration program called Sonarworks, some say it is just gimmicks, coz it uses eq to make my HD650 sounds more balanced. It's actually a plug in for media player. But, the engineer behind sonarworks had done alot of research and calibration to attain that EQ. It cost $99. But for me it's worth for the upgrade. You can also have a look at it.