Oz, I'd definitely say they outperform most tube amps I've heard (especially in terms of bass articulation and top to bottom resolution). And the amazing thing is that you can get them to sound just like a great tube amp if you want to -- simply by using a great tube preamp ahead of it.
I have a friend who uses his NuForce with a TAD-150 Signature with excellent results. In fact, it caused him to get rid of his tube amp, which couldn't keep up sonically.
I'm currently using a Parasound P/LD-2000 into a TAD-150 Signature and then into the NuForce Reference 8s. I know that using two preamps in tandem might seem weird, but I'm actually getting the best sound quality that way. It's my own special recipe that I discovered by accident when I first hooked up the TAD-150 to see if it worked. I found the TAD-150 by itself (though an excellent performer) to be just a touch soft and laid back for my taste.
The solid-state P/LD-2000 feeding the TAD-150 somehow lets the system maintain its excellent dynamic kick while also creating sweet, airy tube sound. It's fantastic on vocals and guitar and the bass totally comes alive. I consider this combo the best of both worlds. I'm using SignalCable's excellent Silver Resolution interconnects to connect the chain and it's pure magic.
