In fact, I have. This substitute was popular for a very narrow slice of head-fi history, in the mid-2000s. The Singlepower OTL tube headphone amplifiers were extremely popular at this time, and they offered many different configurations & customizations, including adapters for various tube substitutions. These amps used a trio of dual-triode tubes, originally all 6SN7. The first tube was input/gain. The next 2 were both for outputs (white cathode followers?). Either the design was originally intended for maximum flexibility, and/or it’s just a fact most of these tube circuits tend to be fairly tolerant of this. The intention for most of us was NOT lowest distortion - it was to achieve sweetest sound and synergistic pairings for the top headphones of the day (each of which had its own strong voicing and quirks).
What we found was that 6SN7 were a little weak for driving headphones directly, especially with headphones of lower impedance (below 120 ohms). A pair of 5687 output tubes via adapters was one of the early successful subs here. I had these, and it rendered way WAY more slam, weight, and dynamics compared to 6SN7s, at the expense of shrinking soundstage a bit. In fact some of the early amps shipped this way were labeled as the "SLAM" version. In a preamp I suspect the effect will be reduced in magnitude, but would still grant some of this “character”.
Pretty sure these 5687 draw a LOT more heater current (they run hot as hell) so your amp will need to be able to safely supply this. Also the optimal operating points are likely to be different, so standard disclaimers apply. BUT it worked quite well in these amps originally designed with 6SN7.
As experimentations evolved, most of us ended up with 6BL7GTA or 6BTX7GT tubes on outputs (no adapters needed) and 2C51 /396A (with a different adapter) for the input tube. But I still fondly remember that initial 5687 sub, and how much I enjoyed the sound! 7044 was also a popular (if harder to find) sub using those same adapters.
There were also crazy "XLR" versions of these Singlepower amplifiers which doubled up to 6 tubes. I’m certain they were far from accurate, but man they sounded AMAZING with Sony MDR-R10 or even a pedestrian Sennheiser HD650.
