First off, it's awesome you're getting your kids into good music. If there were a few million more parents like you, we might have prevented Bieber Fever. Unless you're into that. In which case, ignore the advice below and go buy two Bose Wave Radios for each room ;)

I think the first thing you need to do is determine your budget. That might help you get better responses from these forums as well.

I say this because, without knowing your circumstances, my stock answer to your question would be to avoid buying anything without hearing it first. New or used. But especially used. 

That said, you may not have the time, money, or resources to visit a local dealer and listen or troll garage sales. In which case, rolling the dice on a piece of good used gear online is not a bad idea at all. There are some great deals out there. Check out Audiogon. You'll have a better chance of finding something nice when you're buying from the audiophile community who tends to be OCD and takes better care of their gear. 

Knowing your budget is also important because the type of vintage gear you are thinking about is definately a little more up-market than your standard Circuit City fare of the period and has held it's value better. Those 90's Adcom amps go for $200+ used. Add a pre-amp, another $200 or so. A good, used H/K receiver goes for maybe $200 Then, two pairs of speakers for each room $200-$300 a pair (and I definately think you should spring for new speakers). So... you're looking at maybe $800? Add to that, if you're only planning to play from iTunes, you will definately want a DAC for your PC, or, a dock/DAC for your iDevice. That vintage gear won't have the technology to decode digital signals. And trust me, you're going to want to do that. (using the PC/iPod mini-jack would render the bucks you spent on nice vintage gear pointless.) So add another $130, at a minimum.

Still with me? If, like you say, you are not trying to get the best sound possible - for that amount of money (let's say $1000 bucks), you could buy a new decent quality receiver or an Integrated amp with mulit-room capability and a DAC built-in for your iTunes, plus two pairs of nice monitors. It may not be the absolute biggest sonic bang for your buck like the used Adcom gear, but at least you'll have a warrantee/etc and the modern features you'll need.

I've found that going the used gear route pays off best when you're a.) working in a no-budget situation. or b.) buying very high-end gear that would otherwise be unafforadable to you brand new. I think you may find that for the money you'd spend on 90's mid-fi gear, you could get something brand new that would suit your needs better.  

I would be very happy to buy SOME 1990's gear...amps, preamps, cables, etc BUT, I would stay away from speakers (which can and do wear out) and anything digital as it will be hopelessly obsolete.

Okay, so I'd say my budget is ideally under $1000, but I could find $2,000. Again: larger speakers for our great room (600sft), and small (bookshelfs) for our kitchen (something that can be hidden among other nicknacks.)

Can I get something equivelent to KG4 quality for those prices?

I was thinking something like a Marantz 2252B vintage receiver (or a used Harmon Karden) and some nice speakers.

Thanks in advance for the recommendations!

is not an early 90s piece. It was manufactured between 1977 and 1979. I'd check into the Audio Karma forum for advice on vintage gear.

http://www.audiokarma.org/forums/index.php

If you wanted to spend around 500 bucks or less... With some homework, you will get more bang for your buck from the vintage pieces you've mentioned. (of course you could spend 500 bucks at Best Buy for all new equiptment, but i don't think that would make you happy)  

And you're definately safer going for a vintage amp vs. vintage speaker. Take it from somebody who's replaced foam surrounds several times: spring for new speakers, if you can. I don't know those Klipsch speakers well enough to say what their modern day equivalent would be, but maybe dig around on the Klipsch forums.

But for 1000 - 2000, there are many brand new options that would be at, or well above, the quality level you'd expect of the vintage gear. 

The Peachtree Audio products are good. I mention them because you said you're planning on using iTunes and computer audio of some sort. So you're going to want to get a good DAC or iPod dock. Digital anything is a big point against going vintage. A very modern source with not-so-modern amp means you'll need something to bridge the two vs. a integrated with a DAC built in for a plug n' play all-in-one type o' deal.

I've also heard good things about Emotiva. Check out The Entry Level on Stereophile for other ideas. NAD still makes great amps that don't cost a lot.