We tend to get a little carried away in this hobby (understatement of the year, I know). Upgrading your headphones first will probably bring the most dramatic improvement to your system, you don't necessarily need world-class gear in order to enjoy them.
Unless it's a power-hungry headphone like the Audeze and Hifiman planars, or the Sennheiser HD800 (I know people will argue this one, but it does sound best with a hefty amount of current behind it. I've heard it), a modest source chain with a great headphone (e.g. SR225) will almost always sound better than a mediocre headphone (e.g. $40 Sony) backed by an expensive source chain. This is a bit of an exaggeration, yes, but it illustrates my point. My grandfather once said, "If your speakers suck, does it really matter how good your amp is?).
Buy the SR225, and if it sounds good to you, leave well enough alone. We sometimes tend to use music to listen to our equipment, rather than using our equipment to listen to the music.
I prefer the SR225 to the 325, personally. I found the 325
extremely bright, and the metal housings are heavy which makes them uncomfortable after a while.
I use a vintage Pioneer SA-8500 integrated amp to power my HD650s. World class? Wasn't even top of the line when it was introduced in 1976. Does it sound really really
really good? Oh yes
. Best $100 I've spent in a while.