my Talon Khorus sounds bad

Believe it or not they might not be broken-in yet. It can take a hell of a time to break them in. Some people just getting tired and sell them for nothing comparing to the new retail price. The break-in time should be minimum 400hrs(months of continuous playing). If you have some recordings of the ocean waves than play them at the high volume for a couple of days to "massage" the drivers. This kind of CD you can get from amusicdirect.com. Once you play it you can see how your drivers are moving.
If you listen to a heavy metal or hard rock it doesn't mean that for the sound sake you must listen to some XRCD, SACD jazz...
The best sounding rock recording as far as I know is King Crimson -- Larks Tongues in Aspic. You might also want to check your speakers with Queen's Innuendo. Metallica has lots of MFSL stuff and every album recorded great.

300i is great as a Rock'n'roll amp and good enough to drive Khorus. Rega Planet seems to be lifeless for any kind of music either electronic or acoustic imo. It's got plastic body and pocket CD-player mechanism besides. I still do not know how you can tweak plastic bodies. You can literally "downgrade" Planet to a better source.