Do you mean the TU-3 or the TU-3W? The "tone suck" happens because the buffer is not high-headroom enough, and it actually clips.
I don't have the TU-3 anymore. I sold it (good riddance), but I did try recording DI's hoping I could benefit from the buffer, and I could clearly see every guitar that I ran through it produced DI's that looked hard-clipped and/or brickwalled like active EMG's with the preamp. Anything from overwound PAF levels and up. Like, not even super high output pickups only.
Worst. Fucking. Buffer. Ever.
If you run another buffer in front of it, you're just going to raise the noise levels slightly. The clipping is still going to happen regardless because adding another buffer in front does nothing for the headroom issue.