Conrad-Johnson PV 11 preamp tube dilemma

Did you allow enough run-in time on the Morrow cables? If not, the dull sound you are hearing may be the cables.

Regarding tubes...
By changing from Tele’s to Amperex in the line stage you gained a sweeter, more open midrange.
You could try Amperex 12ax7 in the phonostage.
You could also try one of the Telefunkens in the 12au7 position or an RCA cleartop for some top-end extension. The RCA 12au7 Blackplate presents open, sweet, warm sonics and the 5814A is even better with a sense of realism.
If you are using a Mullard shortplate, it may sound closed in and too warm. If you find you need a warm tube that is more open and extended, Brimar is the tube.
For 5751, RCA or GE triple mica are some of the best.
It’s a good idea to introduce only one tube type at a time so you know which tube is affecting the sonics.

Have you checked the info on Brent Jessee’s site?
http://www.audiotubes.com/12ax7.htm

Also, please tell us what amp and speakers you have.

It would also help to tell us what TT, cable, and cartridge you are using.

So many combinations of tubes shouldn’t sound dull.
I’m really wondering if the Morrows need more break-in.

In addition, I read the PV11 manual and your preamp is phase inverting. From your listening position, does the image sound focused or is the soundstage smeared and coming from different directions?
If phase correct, there should be good detail, and the image should should not be dull. 

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.

Nice cart, I like the Grado sound. OK, so you do need to open-up the phonostage with some dynamics and detail. In addition to my advice above, I’ll add that you don’t need a Mullard in your analogue chain.

Some other 12AX7’s; Siemens (good extension, but Tele’s are better), Sylvania (dynamic, transparent and airy on top), Amperex/Philips (same factory, many different choices). There are Sylvania 12AX7 and also 5751’s. The 12AX7WA Sylvania military is a popular tube. Some Sylvania’s can sound bright, so they should be mated with a less extended tube.