Conrad-Johnson PV 11 preamp tube dilemma

One of the MA4 cables is two to three months old.  The other one is new and had ten days factory break-in, but certainly needs a lot more time.  This is the one that caused the problem. I understand it will probably get brighter as it breaks in.  The thing is, the current configuration sounds so good, at least, with CDs, that  I don’t want it to change!
Turntable is an extensively modified Rega RP3.  Cartridge is a typically mellow, but excellent Grado Sonata.  Speakers are modified Dalquist DQ 20’s. Amps are NuForce Ref 9 v3 monoblocks.
I know the amp is phase inverting.  I’ve dealt with it.

uberwaltz,

What prompted me to use a Morrow cable in the first case was that another piece of equipment I had, benefitted from a Morrow product.  
Then, since that cable (between the preamp and the DAC) worked so well, I installed another identical cable between the preamp and amplifier.  That’s when the dullness occurred.  I happened to have a couple of Amperex 12AU7’s tubes lying around.  I knew them to be brighter than the  Telefunken, so I thought I’d give them a try.
Voila!, the sound opened up.

lowrider, 

Took your suggestions for the 5751 (RCA blackplates-triple) and 12AU7 (RCA cleartop Ecc82.)
i spoke to Brent Jesse about the 12AT7’s.  He suggested Raytheon.  I’m getting them.

I will!

Kudos to those who said “wait for the Morrow cable to break in.”
Indeed, the cable changed character and the sound is terrific now. I’m  sticking with the original preamp tubes.
Evidently the tubes in the phono section I bought because they were the cheapest I could find, work wonderfully with my preamp.
I also added high quality power connects to my amps.  That might have had an effect too.

Next time I’ll be more patient.

Thanks for all the suggestions though.