> Can their power improve a lot with a dedicated amp or is it not needed?

Of course i.e. an ADI-2 Pro has much more power, look at the specs.
And besides this it offers more quality and options.

Whether you need it or not you must ask/question yourself. Best would be IMHO to test the ADI-2 Pro so that you can come to a good decision by comparing on your own. I got one and never will give it away.

Here some information about the ADI-2 Pro and related devices from my blog area:

1) my use cases for the ADI-2 Pro besides to use it as pure Headphone preamp: http://www.tonstudio-forum.de/blog/inde … C3%A4rker/

2) you can build nice solutions around ADI-2 Pro, just as example as I regard it as cool:
http://www.tonstudio-forum.de/blog/inde … ort-DE-EN/

3) I fine tuned my Shure Headphones when I compared it with Audeze phones, nice application for the PEQ section of ADI-2 pro: http://www.tonstudio-forum.de/blog/inde … re-SRH-840

4) sorry not in english available, an article about using ADI-2 Pro as a reference quality headphone amp and by testing some nice Audeze Phones (plus a quick comparison to other phones and my shure): http://www.tonstudio-forum.de/blog/inde … ADI-2-Pro/