Open back, yet with bass, is it possible?

It's a really hard question to answer as nobody knows just how much bass you're looking for.  There are plenty of open cans with more bass than an AD900, so that shouldn't be hard.  However, you'll have a hard time finding an open headphone with the bass impact of a good closed one.

Open + Big Beautiful Bass = Pro2900 much much more bass than AD900, but with less soundstage.

Even with the best amp, AD900 ll never have big bass. Its a very good headphone, but if you really want bass...

How about Beyerdynamic's DT990? 

hd600 maybe? 650 if u need even moar bass. nor sure about ad900s soundstage though

Just a few to get you started.

Fischer FA-011

Ultrasone HFi-2400

Ultrasone Pro 2900

Beyerdynamic DT990

Hard to tell how much sound stage you'd give up as I haven't spent any significant amount of time with AD900's.  Fischer's were fantastic for the price though.

HD650 would be almost dead on. DT990 is close.

You could look into a used ATH-AD2000. It's soundstage is supposedly a bit smaller than the AD700/900.

I never found it to be bass light like the AD700. It may have a bit more than the AD900, so it's worth checking into.

I have never listened big bass in AD series. AD2000 is a fantastic headphone, specially for lover midrange and very fast headphone, but if TFOA want a lot of change in bass departement, i think he should quit the AD serie.

Yes, more bass, less treble, slightly less soundstage..., but if you haven't an amp...

Yes, but coming from AD900, the sound signature ll be perhaps a bit too agressive for him. Big bass, recessed mid and harsh treble, not easy to digest after AD900.

Pro2900 is also aggressive, but with a much better midrange, better bass and easy to drive, i think this is the best compromise (for open headphone), but Pro2900 is more expensive than DT990.

Does the HD-598 have more bass than the AD900? Maybe that's a good alternative? It seems that many people who love the AD900 also like the HD-598.

Yes, indeed, HD598 has slightly more bass, but really not much more and AD900 can go below.

Its normal 'cause AD900 and HD598 are very enjoyable headphones for easy listening (pro2900 can be very aggressive, but also so good), but if you want big bass...

AD900 has a better frequency range and can go below. With some songs you can hear differences, but HD598 is slighty more punchy. My bad for my english, I have difficulty expressing my thoughts, lol

HD598 is an excellent headphone, but differences aren't huge with AD900 in SQ.

I bet the AD300 has more bass than the AD900

I'm actually really impressed with this unknown headphone. Only cost me $35 and sounds really good. Even better than the AD700, but not as clear.

I'll probably review it soon since it's now much easier to find and sells for only $65!