I connect it via the 6.35mm phone jack on line 2 which is there as an asymmetric alternative to the XLR input. With the +4dB input sensitivity of line 2 I can be sure that I don't overdrive the input stage. Don't know why they write about an input impedance of 10k, there is a 47k resistor inside before the signal goes to a Hi-Z preamp stage.
The frequency range is ok. 65Hz as "starting frequency" at -10db does not mean that there will be nothing more below but that the sound pressure will be less for lower frequencies. 65Hz corresponds to the C string of a cello and most unmodified Etherwave theremins can still produce that in some way, but it's out of the beautifully playable range. The addition of the ESPE01 module to the Etherwave allows to go down below 20Hz and the MSP3 does still reproduce that, but not longer with the maximum sound pressure. As I said, I do not play punk rock music. But when I play the "Elephant" theme from Saint Saëns' "Carnaval des animaux" which is a solo for double bass and goes down to the lowest (E) string of the double bass (~40Hz), the MSP3 reproduces it in a nice way. And since I have the speaker on a stand at ear height behind me, I can feel the wind coming out of the bass reflex tubes in my neck.
I tried a Yamaha HS5 with a modified Etherwave and with the tVox tour theremin before deciding for the MSP3. The HS series is optimized for being loud, but not for being precise. It simply sounds harsh and metallic.
Please don't misunderstand me, I do not want to sound arrogant, but money shouldn't be an issue. I also have a very restricted budget for my hobbies since I have to bring up 4 daughters. But a German proverb says "Wer zu billig kauft, kauft zweimal" (Who buys too cheap, buys twice). Thus it is better to spend somewhat more on the first run.