Responses from mik971

Preamp recommendations for under $600.00
I would recommend a used Conrad Johnson also per Mofimadness. You know it will match your amp, and they sound spectacular! 
Emotiva USP 1 verse Rotel RC 990 995 or B&k PT 3
The Rotel 995 is an excellent pre-amp and a step or two above the others. 
Sadly missed manufacturers
Enlightened Audio Designs. (EAD) 
Best EL34 reissue?
They are cheap and maybe not as popular, but I have had good luck with Svetlana's. Very clean sounding tube, durable, no complaints. 
DA converter hook up?
You would run a digital cable from the NAD to the DA converter and the analog outs from the DAC to your Acurus pre-amp. Not sure you will get a much better result, your cdp is good and the pre-amp is average... 
Setup advice
I think you are going to hear a large improvement with the Cambridge 651P. It is an excellent unit for the money! Your amp should have plenty of power to drive the B&W's.. 
$1500 for a whole system, budding audiophile
Personally if I started over for that dollar amount I would purchase:Musical Paradise MP-301 mkIII - $400Tekton Mini-Lores $650Emotiva XDA-2 DAC - $250That leaves about $200 to spend on your choice of budget cables... 
Repair-Restoration Tech for Vintage Sansui Amps??
Not sure where you are located but there is a former certified Sansui tech in Verona, WI. I don't think he deals with shipping though... 
Amp for klipsch KG4
If you want good "synergy" I would look at the older Nakamichi receivers. They mate extremely well with Klipsch. Just make sure it is a "stasis" design unit. 
Replace Amp or Preamp?
Tough choice. I am familiar with the ARC unit and it is excellent. I have only heard the Conrad Johnson PV3, PV10 which are both excellent. I would stick with a tubed/tube hybrid pre if it were my choice. The Motif line of pre-amps have good feedb... 
Replace Amp or Preamp?
I also owned the AR LS1 tube hybrid pre-amp. It is an excellent sounding unit!! It did not work in my listening area because it had a stepped volume control so I couldn't dial in the exact volume i wanted. (amp didn't match perfectly) I think you ... 
Replace Amp or Preamp?
If your amps seem to drive your speakers well, pick up a used pre-amp on A-Gon. That way you can sell if for some reason it is not a good match and not loose much. I think it will be pretty easy and you will be thrilled with the results. Again, th... 
Replace Amp or Preamp?
Your Pre-amp is definatley the weak link. I owned the MAC CR10 and it is average at best. I like your suggestion of the tubed pre or maybe try and find a NAD C162 which is an excellent pre-amp for the money. (maybe $350 used?) 
Where do the tubes go?
I would start with a tubed pre-amp or a self-biasing integrated amp. You will be able to roll tubes to experiment with little technical skill. Also Wolf Garcia's suggestion of Jolida which is easy to bias. 
Best budget cables for tube amp
I love Supra 3.4 speaker cables on the (2) tube amps I own. Can be purchased from Madisound unterminated for very cheap and replaced alot more expensive cables in my system. Very natural sounding cable to my ears..