Low output moving coil cartridge

Mapman-your cart output may be similar, but another quick search shows the SP-16 with a 55db phono stage gain. i.e., your preamp is different than the OP's, at least in regard to the gain at the phono stage. I have a lot of experience with the D103 variants with many phono stages, and it's true that 55db is marginal-though workable. 66db, however, is fine and jim-dandy, and I'd wager the OP will have no issues.

What's relevant in your situation is not the power rating of your amp, but the gain of your phono stage. A quick look seemed to show 66db gain, which is plenty for that cart. Should be no problem.

Depotec-

Let's see, top of the head...

a) AcousTech PH1-P

b) Steelhead Phono Pre

c) EAR 834P

d) Klyne 7Px2

d) the phono pre inboard on my BAT 3xi

All the outboards bested the BAT (as to be expected). I kept the Klyne and the EAR, but if I had to have just one I'd prefer the Klyne-but I've yet to try some good SUTs with the EAR, which is supposed to be a big improvement.

In the Denon 103 world I've got the 103, 103r, 103r w/VdH type 1 rebuild, and a ZU 103r. At some point I'll have SoundSmith rebuild one of them, and I also want to try a 'Uwe' Panzerholz body.

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.