Loudspeakers for hifi come in many varieties, sizes, shapes and methods of working. For the DIYer, only a moving coil design is really feasible, with maybe a few more variants for the high frequency unit (tweeter).
For a loudspeaker to reproduce the whole audio spectrum, it must have compromise. No single element can reproduce such a wide frequency range; the requirements of bass reproduction and high frequency generation are totally different. So in the best loudspeakers, multiple drive units are used. Large cones with generous displacements or multiple smaller ones are used for bass generation, and small light diaphragms are used for the high and super high frequencies. Often, a small coned or domed unit is used to cover the mid range, this needs to be neither large nor very light.
Starting in the 1960's, I have been designing and building loudspeakers for hifi, simple closed box ones, and very large ones based on transmission line principles. Although the latter are instantly impressive, after a while, the bass seems a little detached from the rest of the music. This is a well known phenomenon, to do with time delay smearing. It is found in any loudspeaker which allows sound from the back of a drive unit to feed through to the front of the loudspeaker, for example, bass reflex, transmission lines, ABR's, open aperiodics, electrostatics and ribbons, open baffles, back horns, in fact, nearly every design apart from infinite baffle (closed box).
However, having lived with Epos M5's, which are stand mounted small bass reflex designs, the lack of decent bass has been all too telling, and anything present below 80Hz is dealt with by the rear facing vent, so it is producing, rather than reproducing the sound, which is quite artificial. I needed a 'speaker to reproduce the deep bass (down to about 40Hz, the lowest notes on bass instruments like the double bass or bass guitar) without sounding artificial. Around Christmas 2011, I saw a pair of Usher 604's, which had a good review in HiFi World, going for decent money on eBay. I bought them.
Although I have mentioned the drawbacks of loudspeakers that leak sound, I ended up with another vented design. But trying them out in the listening room showed that they needed some bas cut on the amplifier to sound right, so I am wondering if I can convert them to an aperiodic design. But do they sound smooth! Just like a symphony orchestra does in full flow, just sublime. I'll add a ribbon super-tweeter to add back the sparkle that my ears have probems detecting (too many loud PA systems!) and that should be me sorted. The M5's can go for side duty, and I have a set of M5 drive units I can make into a centre channel for 5.1 duty.