I heard a couple of pairs of the YG Carmels at the Newport Beach Show and was very impressed at the coherence and immediacy of these speakers, approaching what I like about single-driver speakers. I think YG did a great job balancing bass response and nimbleness. That being said, I think it is difficult to say whether you need a sub. As I and others previously mentioned, virtually any speaker can be improved on the bottom end with a sub if you are careful, especially since bass frequencies involve moving a lot of air and the (7" ?) bass/midrange driver of the Carmels are limited by physics in how much air they can move. If you design them to move more air then the sonics will be compromised elsewhere, likely in midrange clarity. There are also many other factors such as the size and dimensions of your room, where you sit, the type of music you play, how loud you play it, your tastes in bass, and so on. Personally, I used to deny that I was a bass hound, when in reality I am and love what subs do, especially the well-designed ones from REL, JL Audio, Velodyne, Wilson Audio, and others. I try to dial in mine so that it sounds as if the low bass from bass drums, bass guitar, tubas, bassoons, etc sound as if they are in the somewhere in the middle of the soundfield and the only way you can determine its contribution to the music is to go turn it off to now realize what is missing. This is not what I hear at a lot of audio shows where the bass is turned up TOO LOUD.
If it were me, I would buy a relatively inexpensive but good sub, used on Audiogon. I had a REL Storm III and it was super simple to just set it and forget it. Yes, it was not as good a sub as my current JL Audio, but the JL also took a lot more adjustments to blend it it successfully with my speakers. If you listen primarily to LPs and/or monophonic music, you can get by with one sub as LP bass signals are often summed into mono. With digital recordings, this is not always the case. Also, some people swear by using one sub for each channel and there are some good arguments for this.