Should I stick to the same manufaturer?

What should be replaced first Preamp or cd player or something else?

System:

Speaker Cable- Monster M2.4s 15'pr.;
interconnects- 2m Monster M1000i;RCA connections not balanced- preamp will not accept balanced;
Speakers- Martin Logan Prodigy;
Preamp- Audio Research LS1;
Amp- Mark Levinson No. 336;
cd player- California Audio Lab icon mkii;
Tuner- Magnum Dynalab MD102;