LUXMAN L509X VS M700/C700U

The Oppo is probably not doing a great job with that system as a preamp, a good preamp can make or break a system.

You should borrow a Luxman preamp and see if it makes the system way better. Other good options is a good tube preamp.

Dave and Troy
Audio Doctor NJ

+1 on borrowing / listening for yourself.

I stand by my recommendations, based on where the OP is starting from, but I also think the OP's wallet and ears matter more than mine. :D

I think OP will replace oppo with luxman Dac. Seems most agree all tube set up will be ideal. 
Agree with mastersound recommendation, best tubes I’ve ever heard as well. They are 
very hard to find on a used market though. I’d stay away from integrated if above tube set up is an option. Is audition possible ?

The DA-06 does not have volume control, so some sort of preamp is needed.  Since OP asked about direct comparisons between the Luxman components, OP can also check out audiodrom.net where all the questioned components are reviewed by a group of 3 reviewers. Good Luck!

English link: http://www.audiodrom.net/en/