Go to the Soundstage! website and read the review by Srajan Ebean. Not only is he one of the best reviewers around, but he goes into some detail about the upsampling vs. Nelson upgrades. To summarize, he found the upsampling did very little compared to the better power supply options. I've heard from other DAC designers that upsampling is not that big a deal and can actually degrade the sound if not implemented well due to the added electronics involved in the design. I'd probably save the money and do the other Nelson upgrades and/or invest in a Perpetual Technology P1A that does actual interpolation to 24 bits, reduces jitter, and offers room correction(although I've heard room correction may not work well with electrostatics--see TacT website for details)--now we're talking serious improvement.
I have to say I'm a little surprised you find the Aerius to be harsh--in my limited experience they seem to be polite if anything so I'm wondering if one of your other components might be driving this. Especially since I've owned an MSB Link DAC and it too, at least in stock form, tends to smooth out the top octaves somewhat. Hope this was helpful and best of luck.