If you're undecided, just google an image of the 2100 with the cover off. It's mostly empty inside compared to the P5. And stay away from a used Emotiva USP-1, complaints are the volume knob adjustment is really touchy, and it can sound forward with the wrong speakers.
With this thinking an insignia stereo receiver must sound really good because their chassis loaded with boards, lol... Of course the p7 is going to have a lot more in its chassis than a stereo pre-amp, that thing has circuitry all over the place, 7 channels plus subs all balanced or rca!!!! Thats a lot of wire and plastic in a half height chassis... But I wouldn't compare that to the p5 or 2100, its a totally different machine, one is for stereo and one is for surround sound, P7 would get used with say an oppo dvd player...
And the p5 internals are similar to the 2100's except the p5 has the built in dac...
To me I want the most direct path possible for music, the more "stuff" {ie a chassis full of boards} between the source and the amplification the more your signal is going to get changed... More direct more natural sound... For music, no room correction, tone control , ect... {tone control has to be bypass style, so when its not on it is not in the line}... I like the p5, and I love the 2100, but to be honest they sound the same, if anything the volume control on the 2100 is nicer, the only reason I would upgrade to it is because of the sub controls, I like built in cross overs with dial controls, vs a set at 80hz high pass... But I use mini dsp which is better than both, The op doesn't care about subs, so if he can live with the looks of the 2100 over the p5 {I like the p5's looks} it would be a better choice... But for sound even with subs, your money would be better spent on a 2100, muse dac, and mini dsp than a p5, that will give you a dac, preamp, and digital sub control for around $700 depending where you buy it...
I like the p5 a lot, but I think people get a little too hung up on the preamp changing or making it sound different.. To my ears all preamps I have heard sound the same, as they should, unless they are changing the sound somehow with switches and a volume control, lol....
Think about it, the signal comes in goes through some wires to a switch then to the volume potentiometer and then back out where you plug it into the amp, where can one unit do this different than another, unless you are using tone control? Sure some tube amps out there color the sound, and some pres have built in tone controls that can change the sound but for a decent ss preamp with a short path and tc bypassed you will not hear a difference. In my experience the only people that do are hearing it because they just sold their perfectly working pre for $400 and spent $1000 on one that sounds the same, lol... Of course it has to sound better at that point, its rare for someone to say, yah I just wanted something new, and liked how it looked...
Think about it this way...
buy this
https://emotiva.com/products/electronics/control-freak
and this
http://www.amazon.com/Source-Tabletop-Control-Switch-Internal/dp/B0056DQT1A/ref=sr_1_2?ie=UTF8&qid=1410104746&sr=8-2&keywords=stereo+source+selector
and for $75 you will have a preamp with a super direct path...
Now of course we buy preamps because we want remote controls and fancy looks, but the above 2 items will switch up to 5 sources {if you need more just add another switching unit} and control volume, that volume control is about the best I have ever seen, all aluminum, and silent as are most now a days...
Where some preamps can be better than others is ergonomics, features like remote controls, triggers, mute, balanced in and outs, cross overs, tone control, pass through, dac's, phono preamp, balance control, ect. and how they sound when changing the volume, you don't want to hear static as you turn the knob, you don't want "pops" as you switch sources, no mechanical noises, no fans, ect... They will not change your sound in any other way, no warmer presentation with air head highs, lol....
Pick the preamp you like the looks of, price, of and features of, then call in your credit card and enjoy the music that most likely will only be changed by your speakers...