I have a SP preamp and a D50. The D50 has an excellent preamp. I don't know if anyone has directly compared the D50 preamp to SP or CA preamps. I have the SP-PREAMP-3 which has 0dB, +29dB, and +50dB. It is very clean even up to the highest setting. The advantage of a SP preamp over a battery box is that you can always switch the unit to 0 and use it as a battery box only and if you need that extra boost you can switch to a gain setting and get a good, clean boost.