If you're recording soft synths, are you outputting them from the computer and running them through FX and then inputting them back into the computer? Or are those just staying inside the computer the whole time?
It sounds like the bigger issue is the string instruments. There are a lot more variables with recording acoustic instruments into a computer. You have to worry about microphone, preamplification and AD/DA conversion. Your Lexicon Alpha is a mic preamp and AD/DA converter. For recording, it's the analog to digital conversion that matters. For playback, it's digital to analog. So strictly speaking the digital-to-analog converter will not improve your recording in any sense, other than possibly allowing you to monitor your own performance more clearly and maybe do a better job mixing/mastering.
The Lexicon Alpha may not be a high-end recording device, but it's certainly better than just sticking an analog microphone into the mic port on your laptop, and using the built-in mic preamp and AD/DA convers.
Maybe if you let us know what problem you perceive in your own recordings, somebody here can advise you whether your Alpha is worth upgrading, or if your efforts and money might be better spent on microphones and/or effects.
With the soft synths, I usually use just one main laptop and record within that, with the soft synths and FX all in it. Sometimes, I use another laptop with soft synths that goes through some external FX, then through the Lexicon Alpha and into my main laptop.
When I'm recording, I just don't feel like I'm getting a clean enough sound with the soft synths, even with effects off and the volume up enough. I feel like it's just a tad bit off. It might even be my external speakers, which aren't exactly high end but I was thinking it might have something to do with recording primarily through a laptop. I never upgraded the soundcard on it, so it probably is pretty lousy.
Maybe it's just me though.