I agree with 'Almarg', but will make one more point, or maybe try to clarify: many phono pre stages have adjustable gain in order to more closely match a given cartridge's output to volume attenuation. Since, as a general rule, the noise floor will increase the higher the output gain, it's best to keep the output gain as low as possible while still having acceptable attenuation at the preamp (of course, phono preamps will vary as to their noise floor at similar gain levels-some very quiet, some....not so much).
Since the gain at your stage is set, your noise floor will not change from cart to cart-though, as 'Almarg' says, audible "hiss" may increase the further you turn the volume knob. You can try it right now-with no input from your tt, turn the volume knob to '12', listen for noise, now try it at higher volume levels. I'm wagering that you'll hear very little difference, and as 'Almag' said in his initial post, you won't need much more 'volume' to make up for the new cart's lower gain. Therefore, since I'm assuming that there's a (presumably sonic) reason that you're using the smaller amp, you should be fine with it's continued use.
Of course, all of this is academic, since you can simply insert the higher powered amp into your system and compare for yourself. Let us know what you come up with.
Since the gain at your stage is set, your noise floor will not change from cart to cart-though, as 'Almarg' says, audible "hiss" may increase the further you turn the volume knob. You can try it right now-with no input from your tt, turn the volume knob to '12', listen for noise, now try it at higher volume levels. I'm wagering that you'll hear very little difference, and as 'Almag' said in his initial post, you won't need much more 'volume' to make up for the new cart's lower gain. Therefore, since I'm assuming that there's a (presumably sonic) reason that you're using the smaller amp, you should be fine with it's continued use.
Of course, all of this is academic, since you can simply insert the higher powered amp into your system and compare for yourself. Let us know what you come up with.