tube suggestions/ brand new to tubes

Recommend Ray Tubes sold by Apos Audio or Upscale Audio.Have fun

Welcome to a wonderful rabbit hole!  You'll find that the same tubes will sound different in different circuits and different systems,  It's also very subjective. 

As new issue tubes go, I've been very pleased with my Tung Sol ECC803S gold, and Mullard CV4004 (both a 12AX7) for my preamp.  In my amp I've settled on EH12BH7a driver tube, and Tung Sol 6550 output tubes.  YMMV!

Rob, 

I’d give the tubes and the preamp more run in time before I’d gravitate to tube rolling to see how close it comes to the sound characteristics you are looking for. That said, the 5751 has a lower gain than the 12AX7 , they are tube equivalents , so I’d start with a 12AX7 first if you want to roll to see if another type comes closer to your SQ requirements.  I use NOS Tung Sol 5751s in my Line Magnetic 518IA integrated amp, the  12AX7s in it were too much .  On the otherhand, I use NOS GE 12AX7s in my Finale Audio 7189 integrated as the 5751s sounded too soft in it. 

kevemaher mentioned Audiotubes , I think the website is till up if you want to get a read on different tubes per his suggestion; however Brent Jessee sold the business a few months back and from what i can tell the new owner - his name is mentioned on the webpage- isn’t fully up and running. Vintage Tube Services and Tube Depot are two other options, particularly if you are looking for NOS tubes ; Tube Depot also carries new production tubes.  VIVA Tubes is more a new production vendor. 

In my world, it isn’t just the tube or brand, its particular vintage during a long run of production, factory code, or just the circuit, but the voicing of the entire system, which may or may not be made of other tubed components. Juggling that balance involves controlling other variables. I think checking with the manufacturer isn’t just a good idea to be safe, but to get real world equivalents- for example I have an amp that uses the same rectifier but won’t allow a substitution, whereas in my phono stage power supply, I have long used GEC U-52 variants (cup base). That tube sounds better to my ears in my system that an even more rarified GZ34 metal base (Philips-Miniwatt), which has phenomenal bass output but sounds wooly and dead on top. Perhaps, in a different system with different overall needs/voicing, the metal base might be the right choice. For me it is not. There’s also a lot of lore to learn, and though I’ve been messing with all tubes since the mid-’70s, I’m still no guru. 

Other owners, manufacturers are worth tapping for knowledge and experience. Be aware that a lot of "NOS" tubes are pulls- they are graded NOS by measurement, but they have been used in some gear at some point. And, it may be worth exploring both old and new production, since in some cases, the older tubes can get scarce. Thus, my love/hate relationship with the 6h30-

good luck, have fun. 

audiotubes.com previously owned by the great Brent Jesse has a great list and selection, and hopefully the new owner carries on his passion and legacy.

Go to https://www.audiotubes.com/12ax7.htm and review Brent’s commentary on various tubes and sonic characteristics.

You’ll pay a premium for Mullard (worth it IMO), but not the most expensive. RCA triple mica 5751’s are fantastic too. Amperex and Brimar are great. 

Implementation in the gear matters, so the rules are fluid. 

Note that some NOS tubes are not branded or labeled as expected, for example I just sold a pair of Mullard’s, one is branded "Heath", and the other is Mullard, but they are both the exact same tube made in the exact same factory.

Personally, I never bought into the idea that there are different levels of new production tubes ranked like American Express cards. 

So the key is a trusted source. If I was still using any tube gear I’d give the new owner a shot. Brent used to have a solid return policy, not sure if that remains but if it does you have nothing to lose.