I think the 12AU7 has 20% the power of a 12AX7 ...The 5751 has 70% of the gain of a 12AX7. The 12AT7 has 60% of the gain, The 12AU7 has around 30% (I think)
You may have 'gain' issues but you won't damage anything. Driving a spring tank too hard won't melt the springs. it'll just sound bad. And driving power tubes too hard may wear them a bit quicker, but they can only put out so much power. They will limit and won't won't hurt the OT unless the amp was badly designed, IME.Yes and no. One can certainly sub lower-gain tubes for higher gain, but subbing higher-gain for lower gain can create problems in some instances. For instance, if a reverb pan is normally driven by a stock AY7, sticking an AX7 in the same spot may slap those springs harder than would yield a pleasing reverb. As well, cascading high gain stages is understood to deliver a lot more high end. If it runs out of headroom, AND you dime things, the output transformer may not be spec'd for handling that sort of output.
Like they say, moderation in all things.