I would buy a new enclosure before I bought anything !
1.25cf ported for a diamond D3 is off spec by a bunch,the spec is 1.75CF per sub
1.25CF Ported for a 12 is really small for any 12" woofer and if you really wanna get loud you could
have an enclosure made to about 2.25-2.5 CF and shell pound your neighbors walls....
If you want a louder set up first get your box ported to spec and then you can step up the amp
However a D3 will take 600+700 watts a peice nicely and I have seen higher numbers.....

The eD 9.1 is a great choice for 2 D3s,and I happen to have one avalible if you like sitting on my shelf collecting dust.....

Also check your Ohms config to make sure your powering up what you do have correctly and ignore local dealers
sometimes,they come up with the most outlandish lines of BS I have ever seen

 

^It all depends on what youre doing man, just because your box is bigger doesn't make it right^

It's all about what youre wanting from your system. There are tons of questions to be answer before you recommend to someone to build a 2.5 cube box for their subwoofers.

 

I mentioned DA Specs are 1.75CF per driver in lue of the 1.25 the guy is running now..........
Having experimented with DA subs in the past (MD,M6,D6,CM3,TDX,D9) I can assure you that
1.75 IS the superior SQL enclosure for any and all DA 12" subs..........
I did also own a 2.5cf SPL type box I had made for DA12s,I didnt like it much and tossed it to the trash
but it was very loud.....

 

The cubic feet of a box have very little to do with the SQ of a ported enclosure. The primary factor in determining ported box sq is the tuning frequency of the port. A larger box will make it a little easier to implement a lower tuning frequency but the biggest factor is the tuning frequency.

 

true true,but you cant achieve the proper port lenth and FQ etc etc in the incorrect enclosure........
Shoot,If I had 1.25CF ported on them subs Id try sealing them if I could,thats closer to the sealed spec