My vote is for dual VBSS. My sub progression has been Polk psw505 -> Rythmik fv15hp -> Rythmik FV25hp -> Dual VBSS and a Full Marty. I cant imagine skipping straight to the DIY subs from the Polk. You will be blown away.
Run the VBSS's at one of the lower tunes and if you want even more ulf later add one of the Martys.
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Equipment: Yamaha 3050, Oppo 103d, DIYSG HT12s (L,C,R), PSA mt 110s (SR,SL), 2 VBSS, Submaximus, 4 driver BOSS build, umik-1, Epson 3700