Herb, you will want to pick up a Marantz 7t or a 3300 or higher preamp, and then keep an eye out for the close to matching era 120, 120B or 150 Oscope tuner. They will fit better with the 510 than your 125 tuner.
Is yours a 510 or a 510M amp? Either way, you got the king of the hill there.

Give me a ring, I know a little about Marantz.
