A large part of the sound is shaped by the preamp... because this is where the frequency response is initially shaped, where your EQ occurs, etc. Extra sound changes happen at the poweramp stage ONLY if the poweramp is a tube amp; in which case, the compression of the tubes will begin to "color" your sound as you crank the volume. In other words... at lower volumes, your sound is 100% preamp. Only once you reach large volumes (where tube saturation bleeds in) will you get sound coloration from your poweramp. Note that in a solid state amp, it doesn't color the sound (unless the amp designer designed it to do so... usually if this is done, it will be to try and mimic tube saturation). Also note that other factors play into your sound: which cab you're using, which speakers, where you're playing (acoustics), etc.
Due to the above... you'll get near 90%+ of the Recto sound with just the Recto Pre (combined with some other amp/cab/etc). However, it will not be 100% Rectifier sound... but will be very close.
The 2:100 combined with the Recto Pre is essentially an exact duplicated Dual Rectifier in rack form. Specifically... if you hook up a Recto Pre + 2:100, the internal circuitry (especially for Modern mode) will be 100% identical. In other words, you'll get 100% Dual Rectifier sound from a Recto Pre + 2:100 combination, because that is how the internal circuitry is configured.
If you're using the DIRECT OUTS with just the Recto Pre (and you flip the switch on the front to "warm" I think it is, whatever the other choice is from "live")... the Recto Pre is designed to and should emulate a Dual Rectifier connected to an oversized (typical) Mesa Recto 4x12 cab. The idea behind the Recto Pre... is that you can record and play without going through the hassle of hooking up a cab, setting up mics, etc.
Note that the controls on the front of the Recto Pre are VERY sensitive (like all Mesa amps). What you put the knobs at for Recto Pre+2:100+Cab+"live" setting WILL NOT sound the same as Recto Pre+"warm setting">direct recording/playback. You really have to invest time understanding the controls and tweaking them to find the right sound. In my opinion, the Recto Pre is perfectly capable of being used direct. Even more so, there are no issues live (Recto Pre+2:100; this is a Dual Rectifier in disguise!).
The problem, as others have pointed out, is that most people who test out the Recto Pre are unaware of these differences. They assume what works for "live" will work for "warm" (their fancy word for "recording mode"). So many quickly conclude its junk. Additionally, many don't understand Mesa controls. As others note, you can max bass and treble and drop mids on a typical amp and find "that sound" you're looking for. Do this on any Mesa and you'll get horrible sound (typically speaking, there are exceptions a la TriAxis = Metallica lol). On a Mesa amp, particularly the Recto Pre (since that is the amp of discussion here), none of your controls should ever be at or near "zero" OR "max"... someone in between and closer to middle will always be better. And unlike most amp EQs, Mesa's are typically different... in the sense that "bass at 12:00" doesn't mean much; how much bass you actually get depends on how all of the other settings are set; they interplay together!
My overall thoughts: if you like the Rectifier... want it rack mount... get the Recto Pre. If you're more into playing live, you should also get the 2:100.
I purchased my Recto Pre in ~2004 or so for what must have been close to $1,300. I've had ZERO regrets. I have a 2:100 and love the combination. But I've BY LARGE used the DIRECT OUTS in "warm" mode way more than live and I'm happy/impressed with it. Hell... I usually have my rack connected into a sound system to play guitar at home nowadays lol. My ending thoughts are: don't buy a floor model. Buying used isn't bad... but I do believe those prices are closer to ~$700.