I'm inclined to agree, there is no reason why your current system should not sound good, so until you understand how and why it does not, you are just throwing good money after bad.
On one hand you say the Diamond 121/122 don't have enough bass punch, but then you (sort of) criticize a similar speaker (155) that have stronger bass. Pick one or the other.
As to adding a Sub, as I mentioned before, adding a Sub to a stereo is not the same as adding a Sub to an AV Receiver. With an AV Receiver you have the advantage of electronic Bass Management for BOTH the Front and the Sub. In a stereo system, with one or two expensive exceptions, you only have the controls on the Subwoofer, which have no control over the Front speakers.
In essence, the Front and Sub are going to run in parallel, and it is your job, by tweaking the Sub controls, to best blend the Sub with the Front speakers. That can certainly be done, and people certainly do it, but you will not get the same smooth perfect integration results that you would with Electronic Front/Sub Bass Management.
Likely in blending the Front and Sub in a stereo, there will be a bit of overlap between the two, and perhaps your taste is not for perfectly blended. Maybe you want stronger bass. That does not seem to be the case, rather your indications are that you want a very balanced sound. The best you can do is hear the Diamond 155 for yourself, and see if it fits your needs.
In my opinion, you will get a better more balanced blend with floorstanding than you will with Front/Sub.
But, as has been said, until you know how and why your current system is not serving your needs, it is unwise to spend more money until you know what the real solution to your problem is.
Steve/bluewizard