Buy sub later
1) you may not want or need a sub
2) you may have greater demands for one higher output, bigger driver, more powerful amp, features like high pass, peq. Movies demand a powerful sub
3) you may prefer floorstanders over standmount for 2 channel therefore not needing a sub.
4) you could prefer ported over sealed, or if sealed Matt need to get better spec to similar rated ported due to how sealed is.
5) put the extra cash into speakers (or amp) so you buy bigger, better sounding, better build speakers,more powerful amp/ features.
In a 2 channel system I prefer bigger speakers and no sub. I use kef reference floorstanders in hifi with svs 12" sealed box 400w but the sub is not needed, if you went for those celestions they'll sound better than those Dali by miles build quality ten times better, you won't need a sub with wider frequency response, and easy to get setup no sub integration to pull your hair out. Also subs in 2ch can be difficult to set up you don't have bass management so have to send full range to sub adjust crossover adjust the gain. Or send high level to sub then high level out from sub to standmount ( if your sub has high pass, and if you so buy stand mounts this is very useful)
You may want to look around for a sub that has high pass I've used it. Both of my cinema and 2ch subs have this feature as does my outlaw ICBM-1. I. A sub-sat or sub-sat system the sub will pass >80hz to speakers and <80hz goes to sub. Not needed if you have floorstanders but worthwhile for bass shy speakers.
Pretty sure bk sub comes with high level cable?