Hi, meambler, I guess my perspective differs from the others but I think you might find a noticable improvement in sound using a separate phono stage, depending on what you get. The HK 990 is likely a fine integrated amp (based on what I've read about it and having used HK amps in the past) and I wouldn't be in a big hurry to replace it. The onboard phono stage may be slightly less "audiophile" than the rest of the amp but that's no more than a hunch; it may sound pretty darn good as is. But if you're serious about upgrading, read on...
The biggest problem is that there are a bunch of great phono preamps in the $1000 -$1500 price range. Even up to $2000 over the long run. That's the range I'd be considering given the system you're running. The range could drop to $750 and there are even more options that would be worth looking into, but I think your current system could benefit from a quality phono preamp, especially when/if you upgrade the cartridge. (But that's a whole 'nother story. '-) Used phono stages are a low risk purchase in most cases so you could stretch your dollar that way.
You'll have to do some research and ideally you'll have a chance to listen to some examples, but here are the phono stages I'd be looking at if I were replacing mine (AcousTech PH-1P with Michell Gyro SE/SME 309):
Rogue Audio Triton (SS) $1000 (I use a Rogue Stealth with a mmf-7)
Musical Surroundings Nova II $1200 (LOTS of flexibility in this one! I almost bought one before going with the PH-1P.)
Avid Pellar $1200 (Has a slot for custom resistors, like my PH-1P.)
Sunderland Insight $1400 (Mainly because I like Sutherland's designs.)
Rogue Audio Ares (tubed) $2000 (The only tubed preamp on my list.)
LIberty Audio B2B-1 $2000 (My first choice if I had the money)
There are other quality phono stages that I had considered at one time but they have limited options for LOMC cartridge loading and gain so they weren't on my list. The EAR 834P is one of them.
It probably goes without saying, but I caution you about putting too much importance in someone's suggestions. Too many variables to be able to know your system and room acoustics and listening preferences. Suggestions are just a place to start your research. Have fun!
Regards,
Tom
PS: I've used a Jolida JD9 for many years and I like it, but I don't recommend it because it has too much gain. It's just not set up very well for MM and high output MC cartridges. If Jolida reconfigured it to provide a wider range of gain settings (eliminating the attenuated "LO" output) they'd have a winner. Of course they'd probably have to charge more money so it would be competing with some very capable phono preamps. One of Jolida's selling points has been value for cost; that would change if the price goes up much more than it already has.