To answer the OP, preamps are not rated in watts. They "draw" current and watts from the wall (Power in Watts = Current in Amps x Voltage), but preamps are rated by their sensitivity and output in Volts. So, let's say you want a phono stage in this soon to be had preamp, okay ? You do want a phono stage, right ? Phono pickup cartridges, of the magnetic and high output moving coil variety, put out a signal in millivolts (rated as mV for a groove velocity). The Phono stage amplifies that mV signal into Volts, in order to drive your amplifier. Typically, a phono stage magnifies the cartridge output by 40 db or 100x the input signal. Thus, a magnetic or HO MC cart with a rated 4 mV at 5 cm/sec. multiplied by 100 equals 400 mV. Since most power amps need 1 Volt or more for full output rating, the phono stage would be connected to the Line stage, another gain stage within the preamplifier for more "drive" needed for most amps. So, amps have a sensitivity rating of needed volts at a certain impedance. Preamps are chosen to provide the needed gain for signal sources, in order to drive the power amplifier. If the preamp can drive lower impedance amp stages at higher than needed volts, this can be a very good scenario. Such a setup can be very dynamic sounding, with excellent signal to noise ratio. With your efficient speakers, being able to keep the volume control down can yield a very low noise floor...
However, you mention this cool SE amp by MR can be used as an integrated amp. That indicates the MR SE amp might have a gain stage ahead of the amplifier stages. If this is the situation, you might only need a phono stage. Some phono stages have high impedance output, which require a high impedance input at the (integrated) amp or line stage of the preamp (Aux input). Matching of impedances is a concern. Some phono stages have a low impedance output, typically using an additional "buffer stage" to provide the low impedance output. It is okay to connect a low impedance source to a high impedance input, but not the other way around. Many of us prefer active line stage preamps rather than the new breed of passive preamps. Even with a cd player signal, additional preamp gain can be preferred.
If your sources of playing music are solely digital (why ?), you might only "need" a passive preamp, which has switching of inputs and a volume control, although many rave about an autoformer or transformer/volume control included in their passive preamps. With a large record collection, I am very sensitive about the inclusion of a mono switch and/or a stereo reverse switch within my preamps.
Knowledge is power, so, research more on this subject before "jumping in" at anyone's recommendation. You must decide which type of preamp you need or could prefer. Then, hopefully you can hook up with a fellow audiophile or so-called audio store/salon, nearby for some listening sessions. Make sure you bring some favorite recordings which you use as a reference. The fun of audio is always eventful...
