How Will A Quality Preamp Help

Honestly speaking, I think your amp is your weak link here.  Don’t get me wrong, I am actually a total Krell fanboy.  I absolutely love the Krell analog audio circuit and it’s the best thing since sliced bread! lol.  Your Krell S1500 uses that specific variant of the Krell discrete Class A input/driver stage.  However, like many other Krell devices at this level, the low voltage power supply for the input/driver stage is SEVERELY undersized.  Even though the amp itself has enough power to push 300 watts a channel, the input/driver stage dictates the sonic signature.  When the power supply is undersized for that input stage, the result is a very bright/harsh with thin body and weak bass.  I have done this on more than one Krell preamp type device where doubling or tripling the power supply capacitance turned the common yucky Krell signature into something magical with full body and powerful strong/deep bass.  I believe Krell has detuned these lower level offerings on purpose so that they have a reason to charge insane amounts of money for their top-end stuff.

Getting something like the Mark Levinson 326s preamp will definitely be somewhat of an improvement over the Yamaha processor, but you are never going to be work around that problem in the Krell amplifier.  It might even make it worse because the resolution of the 326s would be much higher than the Yamaha, so you could potentially be revealing the Krell flaws even more so.

You could get an integrated amp if you wanted, but it is not going to sound as good as a really nice preamp/amp combination.  Some of the upper level Hegel integrateds come to mind.  The Hegel H360 would be a great choice and it would beat the crap out of the Krell S1500 in terms of sound quality.  An integrated can be a good solution if cost is a factor.

That being said, getting a ML 326s and pairing it with something like a Parasound A21 or the Parasound JC1 monoblocks would sound absolutely stunning!!!  Another excellent preamp is the Krell Illusion or Krell Phantom I/II.  DO NOT get the Phantom III because it suffers from the undersized power supply (like I discussed above).  There’s a Krell Illusion II on audiogon now for $3400.

If you are intending on ultimately getting separate preamp and amplifiers, I would choose to upgrade the amplifier first in your situation.

It's good that you stated what kind of speakers you have.  The Rockport speakers can be touchy because of the beryllium tweeter it uses (as well as the carbon fiber drivers).  Because of this, you might need electronics with a more neutral and controlled sonic signature.  I think the Hegel could end up being too bright and exacting and will cause the Rockport to be bright/harsh, even though the Hegel excels in bass.

If you are looking for an integrated, I would look at things like the Pass Labs, Accuphase, McIntosh.  The Pass Labs will be the best resolution out of all three.  The McIntosh would have excellent bass and soul to the music.  The Accuphase and McIntosh could end up being too warm and dull sounding for your tastes.  There is a Pass Labs INT 250 on audiogon for $8995, but that may be too high for your budget.