Help me with some Audio filters

I've been down this road before, and it is virtually impossible to make changes to raw audio to make an electronic piano sound more like a real one. Especially if you are limiting yourself to Audacity.
I find that the most significant contributor to making something like this sound electronic is the attack portion of the sample. Most electronic piano sounds have a rather limited range of samples for different velocities (say, 4 samples per key distributed among a range of 127 velocity values for that key).
For a more realistic sound, you would literally have to vary the characteristics of just about every recorded note.

Additionally, real pianos exhibit sympathetic resonance, something which most electronic pianos do not model well, if at all. I'm sure if you were devoted enough, you could probably write your own audacity effect plug-in that would model sympathetic resonance, but that's probably a lot of work.

Alternatively, you could try and midify that recording and plug it into Native Instruments' Akoustik Piano. I've been really impressed with the quality of their samples.

Jam it back in, in the dark.

Originally Posted by Zergrinch

Assuming I ditch Audacity and declare that the sky's the limit, what's the best way I can modify this recording?

Celemony has a fantastic line of high end audio manipulation products. Currently they're working on software that will enable you to manually manipulate the details of any specific note in a recording. (perhaps even replace one sound with another) Sadly, I don't think its out yet.

Nonetheless, they have an audio-to-MIDI feature in their Melodyne line that should be pretty top notch, especially for something as simple as solo piano. Coupling that with my previous suggestion of NI's Akoustik Piano and some custom manipulation of the MIDI data, you can probably achieve what you want.
If you can't find a torrent for NI's AP, I have it, but its also ~20 GB for the full installation pack.

There's nowhere I can't reach.