Did you actually AB OTL vs SS. My BHCs kills the Ragnarok 1, Bryston, and Violectric v281 on the 600. Not even close.
Very similar to me. In fact besides the years it's the same - until 2019 when I heard my 600 on the BHCs, then bought the kit and built it.
I find the 650 thick and veiled vs 600 on SS. 650 is better on OTL, but it's not the 600.
* I’ve said it a million times, and not everyone agrees, but: In my opinion, with a transparent, basically neutral headphone like the 600, how you feed it is critical. I’d rather see you stretch and get a better amp now - it can save you a lot of money and grief in the long run. If you go with a cheap amp that isn’t stable into a dynamic, 300ohm nominal load, you are asking for thin, bright sound, lacking in foundation. That’s not the headphones- that’s the amp not being able to properly drive the headphones. I know it’s an old school ‘stereo system’ way of looking at it, but two channel stereo is my primary experience and carrying that garbage in-garbage out theory over to headphones has worked for me. And don’t misunderstand me - bagwell, Jonathan c, and BobG are all experienced audiophiles who, like many, many others, have found that there is much pleasure to be had with the 600s. The fact that we all have different ideas about how to go about doing that means only that there is more than one way to skin this particular cat. The key is to be open- minded and don’t expect plug and play - it’s going to take a little effort to get it right, but it’s worth it, so have fun with it.
Yes, patience. I've tried a lot of mods - some work, some don't. For instance stock the HE-500 and HE-6se v1 are not as tonally pure as the 600. After the mods and a bit of EQ both are far more satisfying than the stock 600. My 600 isn't stock (cable, changes to rear padding, cut bass resonance) and my OTL elevates it. I like it more than my HEX v2. But not more than my modded 500 or 6se. Very solid considering all the cans I've rejected.