As the arrow is much more expensive I'd like to point you to the jds labs C5. It is more than $100 but about half the price of the arrow. And it is extremely reference quality for a portable amp. I use it with the 232 and it's the best they've sounded. It doesn't make them have any more treble, but it opens up the treble "details" by revealing more space between instruments and such. highly recommend it. The e11 is kinda crappy because you can't use it while it's charging from what I understand. If that's important to you (i use my c5 charging sometimes). The zo is the lowest quality of the bunch. While it isn't "bad", it fluctuates the entire frequency spectrum if you use any bass boost settings other than none. :-P Some people like what it does, but the 232, although V shaped, is very smooth throughout the frequency spectrum. The zo would add unevenness to the sound. Good or bad would be the opinion of the listener, but definitely less "accurate".
I'd go for an e12 if price is absolutely critical. But I recommend going up a step to the more reference jds c5. It has a slightly more consistent frequency response and specs are excellent on the C5. Channel imbalance is 0, volume steps are minute, bass boost is theater like (i wouldn't use it for the 232 personally), custom cases can be engraved, etc. etc.
Just my two cents. :-) John at jds is awesome too. Really top notch customer service straight from them. Literally.
Great, thanks for the suggestion! I'll look into the JDS Labs C5. I'm not sure If I'm willing to shell out the money for an Arrow 4G since I just spent a good chunk of cash on a pair of PFE 232's.