don't think a sub for the BX8 will tighten it up.
very few subs are "tight" and generally used for 60-80hz and below settings/crossover. (the rumble and film explosion stuff)
i have a sub on, maybe a check once i'm 75% ready to burn one. like having a pc speaker set, for checking the mix.
but with your cash situation, <30days BX8's... go get some others and try...whats to lose!
IMO, you should be happy with your monitors for at least 6months!
now if they just sound like sht to you..by all means get some different ones, you shouldn't have to fix them already.
how did I choose??
i record Hiyan Yodeling. This is just me and a SM7 with a Joe Meek Channel strip into a 2488. Passive JBL Control 5 with Tascam PA150. (and a JBL sub only for checks, last 25%)
Playing my Hiyan Yodeling Mix and adding a heavy reverb it sounds like your on a mountain in Sweden, if done correctly.
A/Bing speakers this helped drive my choice...i could hear more! not only more reverb detail, tails of the fine mountain reverb sound fading off....into the sunset fog.
but my dog would start howling with one set...so i knew!!
one set was probably hitting like 30Khz!! "That's IT, those are the ONES!!" i said to myself...awesome!
do you have a dog?
hehehee, good luck & have fun