Recently I've been learning about using different op-amps in audio devices. Swapping in alternative op-amps is like "tube rolling" with vacuum tubes.
With the TI TL072CP op-amp that was stock in it, my Tube Works 4001 DI sounds very good. I have read some players describe the sound of this model as "huge". photo of the stock TI TL027CP op-amp in the 4001 DI
Tonight I swapped a Burr Brown OPA2134PA op-amp into my 4001 DI, replacing its stock TI TL072CP op-amp.
Burr Brown op-amps are widely considered to be at the high end, whereas TL072 op-amps are usually considered to be serviceable. But in the right hands, and to the right ears, any sound can be good.
With the OPA2134PA in it, my 4001 DI sounded very different: it made much less low frequency output than the TL072CP, and it sounded less harsh and aggressive overall. My impression was that low frequencies, especially, now sounded more focused, relaxed and natural.
So I believe that players' description of the 4001 DI as sounding "huge" was in part due to its stock op-amp boosting low frequencies.
As in other instrument audio applications, I like the Burr Brown OPA2134PA op-amp in the 4001 DI for its relaxed, open and honest-feeling sound. photo of the Burr Brown OPA2134AP op-amp in the 4001 DI
Soon I will upgrade all the capacitors in my 4001 DI. If nothing else, the original caps are now ten years old; but I think better quality caps will improve the audio in any case. Upgrading the resistors to metal film ones is also an option, but its a lot of work, and I haven't yet noticed my 4001 DI being too noisy.