I agree with Unsounds point, it seems you need another amp to drive the Thiels to their best - it is a great amp, but it really is not great at delivering current, a bit like a tube amp, and the Thiels need current and more watts to sound their best, changing preamps is not going to change that.
Preamp with Aleph 5 and Thiels
I have an Pass Aleph 5 amp and Thiel CS1.6 speakers and using an Aduio Research SP9 mk I but with upgraded caps and diodes. This preamp sounded fine in an older system but I was told the impdeence of the Aleph isn't a great match for the SP9 and I find that I would like more warmth, more "tube magic". I know the Thiels don't match well with bright electronics so... do I go for an ALeph P preamp? I am very intrigued by an ARC LS 5 or LS 22. I don't need phono. Any help on what preamp might work in this system?