I'd go for a pre rather than ditch the B&K. Whilst it doesn't double power into 4 ohms, it can still deliver 200 watts into 4 ohms which is plenty for the Thiels. They also have decent peak to peak current, so will work well into the lower impedance speakers.
As for recommendations, I agree with the NAD and Adcom recommendations above or possibly an MF A3 pre. Another possibility - I notice there is a B&K Pro5 pre + ST202 listed for $500 - you could get this combination and biamp with your current B&K - no question marks on the power in that combination!:)
As for recommendations, I agree with the NAD and Adcom recommendations above or possibly an MF A3 pre. Another possibility - I notice there is a B&K Pro5 pre + ST202 listed for $500 - you could get this combination and biamp with your current B&K - no question marks on the power in that combination!:)