You won't get envelopment with standard near field speakers and placement therefore the second subwoofer is not essential in this regard.Firstly, if I have two subs, one for each monitor, and placed near them, should that solve directionality issues?
Placing the subwoofer next to one of the speakers or between both is the best placement.
The placement closer to the listening position will only decrease the influence of the room by a smaller amount, since the wavelength are to long so that the room modes and reflections are still very relevant.If a sub is close to your listening position, does that remove or reduce the impact of the room in what you hear, in the same way that having monitors in near field does?
A near field dipol subwoofer is the way to go to reduce the influence of the room by a large amount. There is no such commercial design out there that I am aware of. So DIY is the way to go here. The simplest design is a naked driver which has to point at your listening position. Have a look at diy audio forums if your are interested. Without measurements you won't get good bass anyways therefore DIY is not that far away. The downside of such a dipol subwoofer is that you don't have any deep bass outside your listening position and you need a very big woofer and a very quiet subwoofer amp otherwise you hear the hiss. The placement is also tricky in a typical computer desk setup. The benefit is that you get very very clean and "fast" bass and there is almost no bass in the room therefore you don't bother any neighbors with your bass. You have to be aware that the transition to a standard boxed bass can be difficult since you might hear the degradation in sound quality due to the jump in more room interaction and potential enclosure issues.
The JBL 308 are good speakers with a good tonality but will potentially hiss a bit and they are not "crystal clear" due to a bit higher intermodulation distortion compared with other (more expensive speakers) and they didn't provide the image and other advantages of a coaxial speaker. I personally would choose a coaxial speaker but such designs also have disadvantages and there aren't perfect speakers. Speakers like the JBL308 provide a very good value for the price.The monitor/sub combination I’m looking at is the JBL 308p Mk.IIs and 310s. The sub was designed specifically for use with these monitors, and has an 80hz crossover setting, again recommended and designed for this combination.
Given JBL’s pedigree, and the fact that the monitors (and subsequently the sub) are designed for near field listening, I’d hope they had the crossover perfect.