Does using XLR cables (double voltage output) mean I can use lower powered amplifiers?

No, it does not work like that.

All XLR systems do is provide DIFFERENTIAL signals. This reduces noise considerably but does not change the gain and/or power transfer.

Below is a VERY simplistic explanation. The detail is much more complex but this is to give a very basic idea:

In an RCA cable (or any NON differential system), if you were to feed 2 volts of signal, typically it would be 2 volts on one cable and 0 volts on the other. If there was some noise of say 200 mV, then you may get 2.2 volts and still 0 volts, hence totally passing the noise to the amplifier.

In a differential amplifier, the same voltage will appear in both the cables as 3 volts and 5 volts (difference of 2 volts). With the noise, BOTH the terminals would be raised to 3.2 volts and 5.2 volts. Since the system works on the DIFFERENCE of the voltages, ie, 5.2 - 3.2 = 2 volts, your noise gets eliminated to a negligible amount.

Hence the much better noise rejection of an XLR differential system

Paul McGowan claims that tube amps can't be balanced.

+1 @cakyol

MC, you should change your avatar to a little knitted black and white bear with pins stuck in it…

Hello,
Please match your amp to your speakers regardless of your sources. It should be a slight benefit to have XLR as a volume increase. Put it this way. Regardless to whatever you input into your iPhone it’s not going to power your speakers. I just don’t want you to damage your speakers. I have been having fun demoing the Hegel V10 phono preamp through using XLR to the preamp. I am still happy I have 100 watts A/B to push the speakers. It make the music sound live. The headroom is incredible. For $1500 MM/MC XLR phono stage it’s really good. I am also demoing the Sutherland Little LOCO phono stage. Due to the input being current driven and 0 input voltage you can hook up two MC turntables at the same time. Imagine; two tables and one phono stage. Stereo only, no mono unless you buy the mono version. I actually called Sutherland and received this information last night. 
If you reside in the Chicagoland area you should try them out. https://holmaudio.com/
You should call and ask about their hours or the amount of time you can have something out on demo. I just like that they let you try before you buy. No more buyers remorse. Plus, If you can’t demo stuff how do you know how it sounds in your system and even more important your room. Sorry for being long winded. I just don’t want you to damage your equipment.