I'm not sure you're going to get a huge upgrade from the Rotel preamp, with a $500 used pre-amp. It's possible I guess, if you look at old enough C-J or Audio Research units.
If you had $1000, a C-J PF-R (SS), or a VTL TL-2.5 (tube), if you can find one used, would probably be a huge upgrade. There are also other used C-J tube pre-amps to consider.
If you had $1000, a C-J PF-R (SS), or a VTL TL-2.5 (tube), if you can find one used, would probably be a huge upgrade. There are also other used C-J tube pre-amps to consider.