Bought headphones need to power them.

Sell your Beats.  If there's any headphone on that list that doesn't need an amp, it's the Beats.

I've heard it said that tube amps under $1,000 aren't worth your time. (I believe it was Uncle Erik who expressed that sentiment...?)

Anyway, why don't you A-B the headphones with your computer, then with your HK receiver? If it doesn't make a difference there's probably no point buying a new amp.

If you really, really, really have to buy an amp--and let's be clear, with these headphones you likely don't--you could spring for the Fiio E9 at $130-ish.

Hm. If you're buying one amp for the long-haul, then I'm afraid you're gonna need to increase the budget. $200 will get you entry level.

Also, if you're using one amp with multiple headphones, solid state is a better idea. Tubes add harmonic distortion which may be pleasing when paired with some headphones, but not so much with others--solid states are more likely to be neutral, and not add anything to the sound. 

I agree with Eric C that none of those cans need an amp, including the 325, though it couldn't hurt.
If you can afford a smidge more than $300 once shipping is taken into account then I have one big recommendation, and two sub-recommendations.

For all the phones you have now and for your next planned purchase of the sr-325s a sheer ha-006+ at either the 1x (unity) or 3x gain settings would fit the bill perfectly. Here's a link: http://www.acoustic-fun.com/catalog/product_info.php?products_id=62&osCsid=096e6bfa12485f822236c858d2d202bc

The 3x setting will also allow for enough versatility that you can work your way up to some better (but also higher impedance phones) like the sennheiser hd-650 or the beyerdynamic line.

The ha-006+ is based on Kevin Gilmore's famous 'dynalo' design. One of the very popular iterations of which was the gilmore lite, a very strong and quite popular amp that is unfortunately no longer in production. A search on head-fi about the 'dynalo' or 'gilmore lite' will yield generally very positive results. You may even be able to pick up a used gilmore lite on the for sale/trade forums. As it stands (and this differs slightly with components) the dynalo design is kind of "wire with gain" meaning there is very little harmonic distortion or coloration in the sound, but it will provide adequate power to whatever you throw at it. Also, kind of a bonus, acoustic fun has excellent service.

Sub-recommendations. These will both be solid purchases for any of the low impedance cans you currently have, and for the 325, but if you get anything of much higher impedance you may end up having to upgrade, which will end up costing you twice. These could be easy solutions for you, but they could also just be temporary little band-aids in the bleeding wallet of the head-fi world.

1. CMOY. You can find them on ebay. They come in cool little mint tins. Elegant and simple design. They sound amazing at the price point. Input is 3.5mm, which we all have laying around somewhere. They can be had for as low as $40 shipped. I recommend ebayuser Juice2214. He does great work, has built thousands of the things. Cons: generally they only run on 9volts.

2. Little ebay tube amps. Bravo, Indeed, Muse, the design has gone by a bunch of different names and they're all equipped quite a little differently component wise, but these can be had on ebay for anywhere between 45-100 bucks, you get what you pay for. Just search tube headphone amp on ebay and osrt by price to find the ones i'm talking about. Pros: Cheap. Cool-looking if you like the exposed sides/glowing tube aesthetic. Cons: Not the last word in clarity or sound reproduction, but if you're into the euphonic tubey sound (do you like listening to vinyls?) then this might be for you.

Ultimately I stand by my initial recommendation, I tried to go the cheap route at first too, but ended up having to upgrade later on anyway once I got higher impedance phones. You say you plan to continue buying phones and just about everyone owns a pair of hd6X0s at some point. I say either get a cmoy now and upgrade to a dynalo like the sheer when the time is right, or get the dynalo now and save yourself the trouble of having to upgrade later.

P.S. Don't let head-fi-ers bring you down about the beats, you got a good deal on them, and as you collect more pairs of phones if you find that you don't listen to them much anymore, they'll be easier to resell than the others. The k240s you got, are they the 'studio' (black with gold) or 'mkii' (blue with silver) model? Not that it affects my recommendations but, what's your source?

Edit: spelling

Edit 2: P.S. added.