how important is linestage preamp with vinyls only

I think I would at the least try a few more 12ax7 tubes,or even better try some Sylvania 5751 3 mica black plates in your current CJ preamp and see how much more performance you can squeeze out ..This may be enough for you to keep what you have and save some money for more Vinyl....

The pre-amp's line stage is important and its presence is audible and influential on the signal it receives either from a phono stage (or an outboard phono stage) CDP, what ever. If your phono stage has tubes and your line stage has tubes then you have 2 opportunities to dial in the tone you want. Some times just the tube in the phono stage will do it, some times just the line stage, more often than not both. Operating on the assumption (I do not know about the PV12) I would start by selecting a tube for the line stage that has a general reputation for being neutral in tone, quiet, and rugged. Then I would try to find a tube that produced the tone you want from the cartridge you are using. It is not always easy to get this right so take your time.

A good place to start is to tell folks what kind of cartridge you are using now, what kind of tone you are getting, and what you would like to improve. Also indicate what tube you are using in the line stage and what tone it is producing as it relates to any source you use. With that info I'm sure there are some folks who can give you some specific and knowledgeable advise.

The linestage provides two functions, if you are using an outboard phono stage. It adds gain. If the output of your phono stage is inadequate to drive your amplifier, that gain is needed. And it adds an attenuator in the signal path and possibly bass/treble/balance controls if you want to use such. Both the gain function and the control functions can also color the sound, for good or ill. If your phono stage has a robust output with the cartridge that is driving it, and if it has a volume control, there is no need for a formal linestage at all.

(There is a third issue with impedance matching; the phono stage has to have a low enough output impedance to drive the cables between it and the amplifier inputs, in order to do away with the linestage.)

If your question is whether or not to switch from a full function preamp (meaning a preamp with built-in phono) to separate phono and linestages, I would say that it is impossible to say, since only you can determine what pleases you most. But separate boxes will cost more, require another interconnect pair, etc. Rollling tubes may help, but what are you trying to achieve? Changing capacitors in the signal path would likely help even more than rolling 12AX7s in the CJ.