If budget allows think about a RME ADI2 DAC. It has both tone controls and an adjustable loudness and many more useful features.
Yeah, that's an awesome DAC, but the price is pretty hard to swallow in what ultimately amounts to a desktop setup for pleasure listening. I already went plenty goofy and abused my bank account with the speakers themselves.
I don't know that I'm really after straight tone controls as much as a suitable loudness curve. As I mentioned yesterday, after addressing the fact that Windows was mucking up my speaker noises, I'm quite satisfied now, but would still welcome a bit more bass cushion and presence at lower volumes. Once I have an opportunity to run through MA 1 later this week / weekend I'll see if I can come up with a suitable "set it and forget it" target curve. Otherwise, I am absolutely thrilled with the 150s for nearfield use. In this application they strike me as true reference quality speakers that put the sound quality burden completely on the recording that you're feeding them, and not on themselves.