Schubert makes a good suggestion with NAD. The C326BEE or 356BEE would be good choices. The Marantz PM8005 or PM6005 should work well especially with your Marantz cdp.
You might want to check out the Arcam FMJ A19. It is known to be a great match with the LS50. Arcam used the A19/LS50 combo at shows and What HiFi recommends the match. I've had the A19 for a bit over a year now and really enjoy it.