That's a loaded question, and I hope I'm not out of bullets when I say that it's hard to say:
You have to consider that one of THE most important performance metrics of a phono section is its adherence to the RIAA equalization curve. If you're not too concerned with critical listening, though, I wouldn't worry about it. With all the other attention to detail this receiver has been awarded, I doubt the phono section was added as an afterthought. But I'd still like to see a spec in the manual.
Your AVR has no provision for cartridge loading, but assuming your TT sports a MM, you should be fine.. The "preset" 47k ohm impedance is standard for moving magnet cartridges, but is mismatched for many moving coils. Note, however, it IS nice to have options because loading can fine-tune the sound to your liking.
Listening to analog records is highly subjective, as many factors such as choice of cartridge affect the sound. Instead of a different preamp, why not upgrade your cartridge?
To answer your question, I'd climb out on a limb to guestimate you'd need to spend $1k+ for a no-brainer improvement. That would get you more precise RIAA, better noise immunity, power supply, blah-blah-blah. But I think you'd be happy with bang-for-the-buck thrills for only a few hundred. Check out Audio Advisor for a few examples.
To ask another: What aspect of the phono stage's performance are you dissatisfied with?
Didn't mean to throw a loaded question. Agreed, I bought the x4000 because it is well regarded. I can't say I'm really un-happy with it. But then again I really don't know what better is, been a Denon receiver/AVR person for a long long time. Also been a Dual TT owner for as many years. Have had this cart since the early `80's.
I've been reading about needle dropping as I want to start digitizing my records, so my question was in consideration of buying an outboard pre w/USB output so I can feed the computer. Begs the question, if I have to buy one, what would it take to better what the Denon offers and use it instead for both needle dropping and playing. An up-grade so to speak. I have a Dual CS 5000 with a Stanton 681EEE (MI) /JICO Elliptical so perhaps buying into the $1k quality range wouldn't be so worthwhile for these pieces and not really my budget anyway. A used ADL GT40 is about my budget so to speak.
But Thanks, I do appreciate your opinion Lou.