I personally feel that any signal loss from using the Wadia direct/volume control issues is reasonably equivalent to any affects of the preamp insertion. This is assuming the use of a decent preamp of course. McIntosh has a tone bypass so only the input signal is amplified through the tubes which I find does not compromise the accurate detail obtained with the Wadia combo. Quite obvious is if one chooses to use the tone controls the output is dramatically different than the direct Wadia, hence in my personal opinion blurred.
In my set up the addition of a preamp extended the superb detail and resolution of the Wadia, enhanced PRaT, but additionally by lenghty tube comparison and employing the tone bypass allowed the soundstage corrections. Coupled with the imaging of the Thiels, I can obtain significant orchestral stage presence throughout the room.
I will also admit freely that my original purpose was to improve the low end response by adding the preamp and use the bass attenuation but have stumbled upon the aforementioned results by accident, and of course buying very expensive NOS/NIB tubes. This is also all with budget cables which I hope to upgrade upon.