Help!!! Need advice on Refurbishing my two NAD 2600's and a NAD 1700 Pre amp

I am having my two amps, NAD 2600'S and my NAD 1700 pre amp refurbished. Which parts do I use??? Capacitors, resistors, transistors... I am looking for the best sound, ( full, rich, deep, smooth ) that I can achieve with this old but good equipment. 

Backstory. I have 2 AR TSW 910 speakers which are power hungry. I have some intermittent static in the left channel that has yet to be identified. It has been into the shop twice. I have a new shop that is going to do the work. 

I had a in-home test/comparison of the Anthem STR integrated and the PrimaLuna HP integrated, against my NAD stuff. The PrimaLuna didn't have the power for the bass. The Anthem, had a ton of power, but no imaging. We re-hooked up my NAD stuff, and to my great surprise, it sounded the best overall. So, the third and hopefully final trip to the shop.

I currently am running a Cambridge AZUR 851 CD player, Bluesound Node 2 for Tidal. I'm using a AudioQuest Coffee coax cable between the two and AudioQuest Waters RCA cables between the CD player and the preamp and amps. I am bi-amping my speakers with BlueJean 10 gauge wire. 

Also, my current NAD equipment has no female receptacle for a power cord. Should I just have them install a shielded cable? I am looking for any and all advice!

Musical interests are, Shelby Lynne, Diana Krall, Frank Sinatra, Jazz standards, Dire Straits, Chicago, Boston, Steely Dan, Elvis, Some classical Piano.... Am I dating myself?

Thanks!