Should I stick to the same manufaturer?

I would suggest cables, then CD (I like Levinson digital in the right setup - and use a 390s myself, but would urge you to consider the audio aero with your amp), then pre...

I personally wouldn't do all levinson with the Martin Logan's. A little tube will go a long way in that combination I think. The CD player may be the right spot for it. No matter what, though, IMHO you should swap out the Monsters when you can...

-Jacob

Get a new CD player or an outboard dac (like the Bel Canot DAC 2 or MF dac) if your player has digital outs. Maybe change cable too. The other stuff is nice.

What cables type and manufacturer would you recommend?

For cables:

New School:

Harmonic Tech
Acoustic Zen

Old school:

Transparent

For Cd player, please understand I am biased:

BAT VKD5SE

Good listening to you!

I would look at the simplest and least expensive items first such as the cd player and cables and ask yourself how much you want to spend and what is it about your system's sound that you would like to change ?
The Acoustic Zen Line of cables are fantastic as are the ones from Ecosse , Analysis Plus and others. A new cd player would depend on the amount that you want to spend but to get a nice upgrade you could look at the Rega Jupiter 2000 if you want a nice smooth, laid back and musical player or something like a Creek or Naim player that offers detail and speed. The new Bel Canto Dac 2.0 is another great piece and it offers upsampling . Other manufacturers also have nice cd players such as Arcam and Musical Fidelity to name a couple.
If you purchase a new cd player or preamp from a dealer have them send you some cables to try with your system as they are very easy to return .