Conventional (and popular) wisdom and opinion dictates that sealed 10" subs work better for music than their ported counterparts. Having said that, i have owned a 10" ported sub in the past (the Yamma NS-SW300) and i currently have the REL and while there's no dount the latter is clearly better, I never found the ported Yamma to be wanting in this aspect. But the Yamma was slightly slower to respond than the REL, something that i never noticed till i did a back to back comparison. And once you've heard it, you can't unhear it.
Im sure you're already aware that the main benefit of implementing two subs is better distribution of bass frequencies throughout the room though it also pays dividends when it comes to bass quantity/loudness. From my experience however, when it comes to music, besides even distribution, loudness/quantity of bass becomes a non-est consideration as you do not want the bass to be overpowering the remainning frequencies. As such, I found myself turning down the gain on both subs in order to get a linear frequency response with the mids and the highs (
with a slight bump of around +2-3db over the rest of the frequencies kyunki baby ko bass pasand hai? insert hiding face in shame emoji). I ended up selling the Yamma as it was inconsistent with the tonaility of the REL so despite adding to the overall bass weight and distribution, the combination worked against bass definition and clarity which to my ears is more important. Since there is more than adequate bass reproduction from the single REL in my listening position, i haven't felt the need to implement a second sub though its definitely on my list, albeit on lower priority to other components which i intend to implement as and when possible.
As i see it, if you already have good bass distribution in your room or at least in your listening position and you don't shift around much, I suppose you can dispense with the need for two subs to save a few bucks. But then again, if you're willing to splurge, please go ahead and get two.