OK, so a bit of a backgrounder:
Yes you get Pre Amps, and Power Amps. You also get "Pre-Processors", which are a combination pre-amp and multi-channel (home theater) processor (Dolby, THX, DTS, alphabet-soup-fits-here).
Now, let's look at what you have in the Denon 3700: It is both a pre-processor AND power amp in ONE box. This is what a Receiver is.
The pre amp/processor part of things allows you to select which source you want, adjust "tone", adjust volume, etc. If present, it can also take a very low-voltage Phono level signal (as in ye olde turntable cartridge output), and amplifies it SLIGHTLY to get to the same level as the inputs from CD/DVD, tuner, Bluetooth, etc. (that's probably why they were called "pre-samplifiers" in the fist place).
The Power Amp portion of things takes (low level, low voltage) signal (think of it coming out of the volume control of the pre-amp, sorta like a light dimmer), and AMPLIFIES the living heck out of the signal that is coming out of the pre-amp/processor. Like from 1.5Volts amplitude to let's say 10Volts (in reality it's more). The increased power coming out of he Power Amp is what is needed to drive the speakers.
Now, MLs are generally fairly inefficient - I'm not sure about the newer ones like you have. What does that mean? Well, for a given amount of output power, they will not be as loud as other speakers.
THIS IS DANGEROUS! Not because you'd have to strain to listen.. no, rather the risk is that you try drive a power amp (or receiver, etc.) harder than you should. THEN the power amp cannot serve up the properly shaped audio wave on its output (going into the speaker), and you get what's called a "square wave". Square waves kill speakers. People will say "oh, there are protection circuits and things"... doesn't matter - just a generally bad idea.
So, when trying to match a power amp to speakers, you ALWAYS want an amp that can exceed the pwer rating of you speakers.. sounds weird, but unless you just constantly blast the crap out of your system, it is highly unlikely you will blow your speakers.
Do you research BEFORE you go shopping. In the meantime, just keep the volume on your 3700 at about 50% of max.
My $0.02 - just get a solidly performing, heavy-as-heck (not kidding - the power supplies of Good amplifiers weigh a ton!) solid state amp. I like Bryston - you'd be fine with a 3B ST (125W into 8 ohms, 250W into 4 ohms (like the MLs), or a 4B NRB or 4B ST (250W into 8 ohms, 400 into 4 ohms), but there are a LOT of good amps out there.
I think any amp that has a rating of 120W or more into 8 ohms, AND IS RATED TO RUN 4 Ohm and 2 Ohm speaker loads, should be fine. It should also weigh 30lb or more!
I would not suggest messing about with tubes just yet - get all this straightened out first!
Hope that helps.
Russ