I think that you will get more out of the addition of a QUALITY sub(s) than you will an amplifier change. Keep in mind that this requires more physical room space, greater electrical demand, the aquisition of another amp and speaker cables if passive, the purchase of an interconnect or speaker cables if powered, etc... There is a LOT to think about when adding a sub(s).
Obviously, the biggest difference would be if your main speakers are pretty anemic to start off with. That last comment may not apply to your specific situation with the Sonata's, but i included it as a point of reference.
A high quality sub will help the most and be easiest to blend if you take the following matters into consideration:
You can actively cross over the mains at a relatively sharp slope to minimize their load and excursion. The use of a flexible crossover and "Q" or "blending" control is a MAJOR advantage.
You can run two subs and optimally place them ( not tucked away in the corner to go "boom, boom, boom" ).
Sealed designs are best in terms of being "tight" with excellent transient response but typically require very large boxes and BIG power. Second choice would be a very well designed transmission line, but these are much harder to build let alone mass produce. There may be some exceptions to this ( like any other "rule" ) such as the Linn Isobarik ( sometimes called "compound loaded" ) design. Craig's Vandy 5's use a variation of this design along with active equalization to achieve excellent in-room response.
Vented designs ( ported, slot loaded, passive radiator, etc... ) TYPICALLY suffer from poor definition, lack of punch, thicker and muddier sound, etc... This has to do with their measurably poorer transient response and increased ringing. As mentioned, there are exceptions to the rule IF the designers have done their homework. Vented designs, as a general rule, will show higher levels of output and greater low frequency extension. This is typically achieved at the expense of "speed", "pitch" and "definition" though. As George mentioned, "pitch" or "tone" of bass is also very important.
Driver size, excursion and how it loads into the room DOES matter. Front loaded designs will produce the most "punch" but also the highest distortion and ability to localize. Downloaded designs are easier to place, harder to localize and produce less "apparent" distortion. Unless the sub comes with a pre-mounted loading plate, you can fine tune the amount of output, extension and "pitch" of the bass by varying the distance from the driver to the floor. The down-side to downloading is that the drivers are typically of limited excursion and will suffer very noticeable "suspension sag" with age. They are also more prone to blowing up due to reduced power handling because of the natural "driver sag" that takes place. Dipole radiators ( identical drivers mounted on opposite sides of the cabinet ) can produce notably smoother in room response but are also typically lower impedance. This gives twice the radiating area and excursion capabilities when compared to one driver, but you've also got twice the reflected emf and a low impedance to deal with. The same goes for subs with several woofers.
Impedance of a passive sub should be taken into consideration. A lower impedance may offer greater power output capability from an SS amp, but it would have to work a lot harder ( and run hotter ) in doing so. Keep in mind that a lower impedance also reduces damping factor, making the driver harder to control. Very low impedances or very large excursion woofers require BAZILLIONS of watts to control and work best.
Sub cabinets should be deader than dead when you give them the "knuckle test". Otherwise their cabinets will resonate and "clog up" or "congest" the mid-bass, upper bass and lower midrange region. This makes blending very difficult as you typically end up with a sound that is "too thick" and easy to localize no matter what you try.
Take a look at Scott's ( Sdcambell ) excellent post about the Widescreen review subwoofer "shoot-out". If you can, try to pick up a copy of this magazine as it gives a LOT of data on the subs that they tested and others that they didn't. Hope this helps... Sean
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