I tried hooking up a regular '2-channel' stereo (a NAD amp and some HSU speakers) to my Kawai ES4. Results were not good. Gain was all wrong, it sounded pretty bad.

I understand for about two hundred bucks I can get a little tube pre-amp from places like 'Guitar Center' that I could run into the NAD to get the right amount of gain.

The other thought is just get some powered speakers, M-Audio BX5a's, probably.

Does anyone have any experience with one or the other of these two options? I'm really wondering if the 'get a pre-amp to feed into the amp' is a viable idea. All comments welcome.

Well, I just got the powered speaker home. They sound awful. And they were pretty beat up, obviously not a new pair, so I'm taking them back tomorrow.

Which begs the question, do I want to bother with a new pair?

I'm thinking it's time to save up for a real piano. I think I may have hit the limits of digital keyboards. They just don't sound that good.

If your using an amp only, normally I think you would need a preamp.
Roger's Speakers, have a sensitivity of something like 85db and are bass limited.
The HSU Speaker website I visited, claimed for their speakers that they are very efficient but needed a sub-woofer. If they sounded more than loud enough for you but excessively bright and/or harsh, then I would suggest a subwoofer rather than a preamp to address your problem.
The same can be said for the M-Audio BX5a's also. They have a 5" wooooooooooooofer!

A DP, to sound good, needs stereo and full bandwidth.

How are you connecting your keyboard to the NAD amp? The ES4 has proper line-level outputs, so you shouldn't really need a preamp with the NAD if it's cabled correctly. And if you do need a pre-amp, $200 seems a bit steep. I'm sure you'll be able to get a good quality one for that price, but it's quality will probably exceed that of the rest of the system, and so be wasted

I have a pair of M-Audio BX8a powered monitors that I use primarily with my Mac for Garage Band recordings.

As a test I hooked them up to the Line-Out jacks (using 1/4" mono instrument cables) of my Casio Privia PX330. The speakers sounded excellent, but they are not real loud. Plenty loud for practice or playing solo, but they would never cut it with a live band and drummer.

Keyboards provide line level outputs, so you won't need a preamp.

I hooked up an FP-7 using the line level outputs to a NAD stereo amp using the auxiliary inputs to 1970's Pioneer speakers with 15" woofers.

The sound is amazing. So it can be done with good results. Not sure what your problem is.

none of the speakers he's tried have bass