Finding "your preferred sound" is not easy, and recommending some equipment to someone which is not familiar is not easy either!

Unless you can try out at meetings, or with friend's equipment, you need to read up here in this site, and be very cautious of people who have only listened to one type of equipment (the one they have now!) and is praising this into heaven!

I bought 4 portable amplifiers, and sold/traded them before I found something I was happy with!

I made a loss, made some experiences, and I can mostly find out now what would suit my taste.

Same with IEM's and headphones (but again, you really never know before you try!)

Now my available portable amplifiers here are getting old, but they give me what I like, and i am happy with them.

The cheapest options are very often not what makes you happy!

T-amps and digital amplifiers, I stay away from.

Ray Samuel's amplifiers I like, Xin too !

They have (originally, or slightly changed ) a sound I am comfortable with.

But these are over your USD 100 budget.

From the ones you have already been recommended (and i don't tell you this is your best choice, really!) , I personally would try out the little hybrid amplifier:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250707766699#ht_7245wt_1135

I don't know this one!

I have not really been into vacuum tube devices (or hybrid ones!), but if I was to try out something new, this would be it.

It is not very expensive either!

Tube amplifiers usually have a "softer" sound than others, so in my opinion they will not go well with too "laid back" headphones.

But if asked, I would recommend this type of amplifier for a start.

Good luck with your decision!

how did you try your ad900s with the soundcard if you said it didn't have a headphone jack?

yah thats out of the line out. any of those amps listed should improve the sound