That sounds like a good setup. I've used an E-MU 0404 USB as an impromptu analog preamp, connecting its 3.5mm TRS analog out to the LS-10 line in, and had very good results. However, the 0404 isn't well suited to portable usage.
Aside from the Mixpre, the USBpre 2 (also from SD) and the
Naiant Littlebox, built by a member of this forum, are also good bets. Note that the USBpre 2 also acts as an audio interface (much like the 0404, but in a much more portable package).
I'm awaiting a Littlebox order to use with my LS-10... I don't know when to expect it on my doorstep, but it should be coming soon. If you go the Littlebox route, be sure to discuss your order with Jon (TS user "mshilarious") beforehand by email, since it will be custom-built and there are
many options available. Once it arrives, I'll be in the
team thread with pictures, then, once I actually get a chance to use it, samples.