You are doing well my young friend but you need more overkill and frankenphones.
Before I went nuts I had a SR-007/Blue Hawaii but even that wasn't enough to stop the evil thoughts from appearing, "I wonder how those crazy Sigma's would sound..." etc.
If you can open it up and take pics then we should be able to see what needs to be done. The screws are probably very well fixed in place (he put some adhesive on them so that nobody could open them up) so I'd buy a few 1/16" hex keys since you bound to destroy at least one. Some pliers also help.
If I had the heatsinks and could therefore link the singles into a double unit, then I'd use them but I do have a small stash of 389's so I continue to use them instead. The LSK389 is supposed to be better so I might just use that instead when I run out.
The only hard to source part would be the 2SK389's but I can point you to a source, you can use a LSK389 or just use matched singles which are cheaper and likely better matched.
For 500$ it's hard to go wrong with the Stax 2050 setup, brand new as well. The SR-202 headphone is much better then the SR-303/404 (not the LE version though or the SR-SC1) and the SRM-252 is good for what it is.