There are a number of factors to take into consideration. The two main factors are the physical dimensions of the speakers and the dimensions of the room, with the latter being far more important.
Conventional wisdom is that small woofers can be closer, but large woofers should be further away, and that's how it's normally done.
5" and 6" speakers are usually placed about 2.5' to 4' away, 8" speakers between 3' and 6' away, and 10" placed 6' to 8' away. Large, full-range speakers, which typically have 12" or 15" woofers, are usually soffit-mounted 10 to 12 feet away. These examples are based only on what I've seen in studios, and
not on any scientific principle that I know of.
In practice, there is really no problem setting 8" speakers 3' away or 6" speakers 4' away. Arguments about "letting the bass develop" are bogus in the context of a control room. You don't actually
want the bass to "develop". That's a concept for PA systems, live sound and churches, not for control room monitoring. The whole idea of nearfields is to try and
reduce the interaction between the speakers and the room.
What you
do want is for the outputs of the woofer and tweeter to blend nicely, with minimal phase cancellation. That's an acoustical process, and it does need some air to do it in. But you only need a listening distance that's at least 3 times the distance between the woofer and tweeter. Take a ruler and figure out what that comes to - on my speakers, that's about 18". Double that to 3' as a safe, conservative rule-of-thumb and phase alignment will not be a concern.
Far more important is where the speakers sit in the room. You want them as far from any walls as you can get them, you want them to be laterally equidistant from the nearest walls, and you want to avoid siting them in the dead center of the room or have your mix chair in the dead center of the room. Getting them out away from the wall behind them is very important, especially if your speakers are rear-ported.
Ideally, you want the nearest boundary (walls, floor or ceiling) to be at least 3 times further away from the speaker than your ears are. If pulling the speakers closer to your ears is the only way to get them away from the walls, go ahead and do that.
The OP's problem is that he's not hearing the bass accurately in the studio, and is correct in assuming that speaker placement may help. In the absence of serious acoustical treatments, the best strategy is probably to move the monitors closer (while still keeping them in an equilateral triangle).
Of course, this won't prevent the room from screwing with the sound. Only heavy bass trapping is going to help with that.
As for the lightness in the highs, that could be because the room is particularly bright and reflective, or it could mean a speaker adjustment is needed.
I'd take some measurements first, running white noise through the speakers into an omnidirectional microphone. I'd want to verify that my speakers are reasonably flat above 4KHz. If not, there is probably an adjustment on the speaker to turn the tweeter up.
But if the problem isn't with the speaker, the room is the likely culprit. Some absorption should clear that up, especially absorbers to the left and right of the speakers, and a ceiling cloud above, midway between speakers and mix position.