@jsbw The 12AT7 can handle a lot more current than the 5751 can. So you might run into saturation and/or a short lifespan with the latter.
Another issue is bandwidth, and this plays a lot on how the feedback in the amp will behave. Feedback relies on Gain Bandwidth Product, which put simply is the amount of gain and the amount of bandwidth. When you reduce the bandwidth, the amp will have less feedback at higher frequencies; the same thing will happen if the amp has less gain.
When the feedback is decreased at higher frequencies the distortion rises (this is why power amps are usually measured at 100Hz instead of 1KHz as the distortion at the latter frequency might not look so good). The ear perceives distortion at higher frequencies as brightness, just so you know.
If you really want to see how the amp is affected, run it up to full power with a sine wave and see if it can make rated power, see how its bandwidth is affected and finally see how the gain is affected. Measure its distortion at 100Hz, 1KHz and 10KHz. These things interact- you may not be able to drive the power tubes to saturation with a driver tube that has a significantly higher plate resistance (which a 5751 has)...
I've been accused of making some of the tweakiest amps made- all triode zero feedback OTLs that are fully differential with one stage of gain. With an amp like that you can imagine that there are all sorts of audible changes you can make. But the simple fact is that you have to take the measurements seriously! What you are looking for is for it to measure well and sound good at the same time. You're also looking for a good distortion signature that is innocuous (you can't get rid of distortion; that's always going to be a thing).
So yes, you might be the first to try doing a sub like that.
Another issue is bandwidth, and this plays a lot on how the feedback in the amp will behave. Feedback relies on Gain Bandwidth Product, which put simply is the amount of gain and the amount of bandwidth. When you reduce the bandwidth, the amp will have less feedback at higher frequencies; the same thing will happen if the amp has less gain.
When the feedback is decreased at higher frequencies the distortion rises (this is why power amps are usually measured at 100Hz instead of 1KHz as the distortion at the latter frequency might not look so good). The ear perceives distortion at higher frequencies as brightness, just so you know.
If you really want to see how the amp is affected, run it up to full power with a sine wave and see if it can make rated power, see how its bandwidth is affected and finally see how the gain is affected. Measure its distortion at 100Hz, 1KHz and 10KHz. These things interact- you may not be able to drive the power tubes to saturation with a driver tube that has a significantly higher plate resistance (which a 5751 has)...
I've been accused of making some of the tweakiest amps made- all triode zero feedback OTLs that are fully differential with one stage of gain. With an amp like that you can imagine that there are all sorts of audible changes you can make. But the simple fact is that you have to take the measurements seriously! What you are looking for is for it to measure well and sound good at the same time. You're also looking for a good distortion signature that is innocuous (you can't get rid of distortion; that's always going to be a thing).
So yes, you might be the first to try doing a sub like that.