I've been working on my piano EQ over the last few weeks. I use the stock sound in Notion 4, and whilst it's not the best stock piano sound...once you know how to treat it, it really sounds great.
Now of course you won't have the ability to dial in sustain through the written score like I can...pedal up and pedal down, but one of the new tricks I've learn't to create a rich deep image from a sampled piano is the console emulator. I'm not sure how you do it with Cake's console emulator (sorry cake, but we all evolve), but with Wave's console emulator just pick a console...I've used both the SSl and TG12345 and dial in 3-4 db of drive...all of a sudden the piano doesn't sound limp and lifeless, but like Beethoven...ok maybe hyperbole lol.
Another trick I've been working on is my EQ technique, and again sorry Cake lol, but Wave's H-EQ is great over the top of the piano sound, and even better is the ability to dial in THD. Man once I worked out how to use the THD on the H-EQ, I was able to dial in and out problem frequencies, as well as keep the beautiful forments' of the instument. One of the other cool things about the H-EQ is the frequency analyser which shows you where the problems are. Two settings that I use for EQ and it doesn't matter if you use Wave's H-EQ or Cakewalk's QuadCurve...if you use Cake's try either the pure setting which is vaguely modeled on the passive RS56 if the eq curves are anything to go on, and roll off with a low-shelving EQ between -1&2db at around 450hz...that will get rid of some of the annoying bottom end if this the problem. Alternatively the E type, which has a little bell kick at the point where you want to tame the problem frequency. This can help tame the bottom end but at the same time allow for the tone of the piano come through...depending on the leading tone of the instrument. If, alternatively the problem is the low-mids, use a very wide scoop at around 450hz again, and lower between 1.5db and 3db.
Another cool trick to help a sampled piano shine is to use a compressor of some sorts. Now depending on what type of sound or tone you're after, I like Cake's CA2 leveling amp, which is really easy to set...you wan't about 3-5db of gain reduction and you want to set the level of the CA2 to about -10 to -5 on the DBFS scale. This will give the piano some attack, and sustain, and also help tame some of the problems in regards to EQ. Another trick is a hard compression setting, and Cake again has two excellent compressors for this...their SSL buss emulation and 1176 emulation :). Pick a ratio of between 5 and eight and just hammer the piano through the compressor. Use your ears to decide whether to EQ before or after the compressor.
Here is a piano piece I prepared earlier lol
https://www.youtube.com/watch?v=Uml1fzus5J0 actually I finished mixing and mastering this piece only a few days ago, after about of month of pulling the mix apart and putting it back together again. Due to a similar problem the OP had.
Ben