Hey Spunkey-Monkey:
I have been using a small Peavey Headphone box for many years. It has 4 inputs; don't need any more for my small in-house studio. Works fine; it has a push/me button for stereo or mono.
I tried a newer unit about three months ago but it did all kinds of weird things like left/phone right phone, no good stereo, etc. So I sent it back. Make sure you can "send it back" before you buy or rent one and try it. The Peavey ran about 65 dollars from a local Music store.
You should get some monitor speakers to use to balance your final cut before you mix down. What you hear inside of good headphones is a nice environment but what you hear isn't exactly what comes out on the mix. With monitors, you can set all of the tweaks to your pleaure before you mix. Then, you can rewind, change reverb rooms and tweak and mix again. Sometimes for a vocal client, I will do about 8 mixes with different reverb settings, hi/low EQ, etc. It does take time but the results are usually pretty good.
Happy listening.
Green Hornet