I have no problem with the EA Doubler, though I'm using a magnetic pickup and a Fishman DI, so I don't have issues with the input impedance. I have experimented with going direct into the front of the amp and using the on board HPF, but the signal was pretty low (due to my pickup's output rather than the input gain on the Doubler) and the HPF didn't seem as effective as the HPF on my Fishman. USA cab guru Mike Arnapol was involved with the design of the EA Doubler and I'm sure I read somewhere, probably on Talkbass, that he wasn't happy with EA's HPF, saying that the frequency/ratio/something was next to useless for the double bass.
If I was buying again, I'd probably go with the new Acoustic Image amp. Although I never got on with the AI Ten2 combo that I had, but that was more to do with the cab than the actual amp.