Wide bass sounds impressive on headphones and costs you the most of anything in the mix. It is also the easiest problem on this whole subject to solve, and the fix does not touch the width of anything else.
Why low end is the expensive part
Two reasons, and they stack. The first is that bass carries the weight of a record. Lose some of it and the mix does not just get narrower, it gets smaller, and every other balance decision you made stops being true.
The second is where bass gets played. Almost every club system feeds its subwoofers a summed signal, because one large box in a room is easier to control than two. So the part of your mix with the most energy in it is the part most likely to arrive in mono, in the room where it matters most.
There is a third reason if anything you make ends up on vinyl, where out-of-phase low frequencies make the cutting lathe misbehave and the mastering engineer will make it mono for you.
The fix, and why it costs nothing
Make everything below a crossover point mono, and leave everything above it alone. In mid/side terms that is a high-pass filter on the side signal: the side keeps its top, loses its bottom, and the mid is untouched.
The reason this is free is that stereo information below about 150 Hz was doing very little for you anyway. Directional hearing is poor down there, wavelengths are longer than most rooms, and a listener cannot place a low frequency in space the way they can place a hi-hat. You are giving up something the ear was barely receiving and getting back the weight that survives a collapse to mono.
What to do it with
Some DAWs ship this. Ableton Live has it built into Utility, as a Bass Mono control with its own frequency setting. Elsewhere it is worth checking whether your stock EQ has a mid/side mode, because if it does, a high-pass on the side channel is the whole job and you already own it.
Where there is nothing stock, and that is common in Logic Pro, Pro Tools, Cubase and FL Studio depending on the version, any mid/side utility placed in front of an ordinary EQ does the same thing: split to mid and side, high-pass the side, put it back.
Where to put it in the chain
- On the master, late, so it catches everything including whatever the reverbs are contributing down there.
- On the bass bus as well, if the bass patch itself is stereo. Fixing it at the source is tidier than fixing it at the end.
- After any widener, never before. A widener placed after the crossover simply puts the problem back.
Keeping the width you paid for
The crossover solves the low end and leaves the rest of the question open. Above it you can still have a wide mix that loses body in mono, because content that exists only in the side still disappears when the two channels are summed.
Narrowing solves that too, and it costs you the width. The alternative is to move that energy rather than remove it: filter what lives only in the side and sum it into the centre, so the stereo picture stays where it was and the weight is still there when the mix collapses.
That is what MSBrooo does, with a mono check and a correlation meter on the same panel so you can hear and see the result while you set it.