Upgrading to an external Phonostages for my system

 It does have a phono stage but I believe there is better phono stages / preamps that would provide even better sound. 

Eventually, I will move up from the HK990.  By having a stand alone Phono Stage I'll have more choices from which to pick. In the future I want to move away from an integrated amp.  As I understand it's best to have an exterenal Phono Preamp anyway.

I may sound dumb but what is "YMMV" Been trying to figure it out.
  

Your mileage may very.

No doubt there are better phono stages than the one in your 990, but I don't know how much more you would get out of it by running a better phono stage through your 990 amp.
It would be sort of like putting a $5000 cartridge on your Scout turntable. A chain is only as strong as it's weakest link.

Don't under-estimate integrated amps either. I have been on both sides, and for me, there are some mighty fine integrated amps out there these days. I've owned monoblock amps, 2 chassis preamps, 2 chassis phono stages. Believe me, you can get as many boxes as you want.

I'd have to go to at least $30K in separates to beat my VAC integrated amp. I know, I've been there.  Then you have to throw more money in for the extra interconnects and power cords. I'll take a great integrated amp over good separates any day.

If you are stuck on a separate phono stage, the Allnic H1201 is very nice, and I would recommend it highly. Maybe overkill right now for the rest of your system, but maybe one day you may upgrade your analog front end and electronics to be able to take full advantage of the Allnic.

Meambler

The Modwright PH150 is one of the best phono stage out there. 

It really depends on your budget.  I just upgraded from a Trigon Vanguard / Volcano combination  to a Herron VTPH-2.  The Trigon combo is a great ~$1k phono stage.  The Herron is a great ~$4k phono stage.

You can check out a review of the HK on the Stereophile website that includes measurements.  I wouldn't say the VPI, Ortofon or HK are weak points, but the Stereophile review does imply that the phono stage could be better.  I don't know the speakers, but they are Revels, so can't be bad.

You are looking for an interconnect from the turntable to amp with low capacitance.  Again, it is a case of budget.  Cardas makes some that are very good.  If you don't have the budget, try the Blue Jeans LC-1's.

To be honest, I would look at setting up the speakers first and working on integrating the sub and speakers properly.