Found PHOTO + info for your Sony SS-AV44's....it's a 40-in High Mini-Tower, with about 1.5 cu.ft. (43-liters) interior Volume:
[includes Schematic for CHEAP "First Order" Crossover...such as it is]
[supplied as p/o STRD 5.1 AVR System]
As you said, it has a 5-cm (2-in) Tweeter, TWO 6.5-in Drivers (per manual one is Woofer, second is Mid-Bass) and a Bass-Reflex Tuning Tube.
Suitable Replacement Speaker Kits [incl. compatible set of Woofers, Tweeters and Crossover Network] are usually called "MTM" type [Mid-Bass, Tweeter, Mid-Bass], where the two Mid-Bass "Woofer" units are nearly always identical....if there are any Kits where one is specifically for Woofer Freqs and other for Mid-Bass, I didn't see it. Most of the available "MTM" kits use smaller 4-in and 5-1/4-in Drivers. SO the fol. is one of the RARE Kits that uses 6.5-in Mid-Bass Units that MIGHT fit your Sony Speaker Holes (MAY require some "adjustments"...try to find some detailed dimensions for both candidate Mid-Bass and Tweeter Units).
[XLS-Encore with 1 Woofer actually contains the links to Woofer and Tweeter Drivers]
Note that this kit comes in either "X-CS CLASSIC" SEALED BOX [where SIZE of the Enclosure affects the Low Freq Roll-off point but is NOT Critical to the Design] and additional cost "X-CS MTM" VENTED PORT [Bass-Reflex Tube may require some "tweaks" to fit Sony cabinet] versions. If you don't feel you have the tools (Sound Pressure Level Meter or Calibrated Microphone with Cellphone or PC) and/or knowledge/experience to TUNE the Bass-Reflex Tube Length for the SIZE of the Sony Speaker Cabinet, then select the SEALED BOX version. Since they are only 6.5-in Woofers, the AVR's Bass Management should set them to SMALL, hence Frequency Response below 60 Hz isn't going to make any difference ANYWAY, since Freqs below about 80 Hz are re-directed to the Sub-Woofer....so I doubt that it's worth the extra effort to LEARN how to Tune a Bass Reflex, when those Freqs aren't going to be heard anyway....
Another more expensive KIT from Europe...note Freq Response Comparison Charts....above from GR-Research won't be all that different:
If you Google "Speaker Kits" you will find over a dozen different suppliers....feel free to expand your search.....
Note that SOME of them can get CRAZY Expensive:
IF you want to PICK and CHOSE your own components, esp. if you want to replicate the Sony's 3-Way Woofer/Mid-Bass/Tweeter Configuration, then you'll need to know a LOT MORE about Speaker Design....and there are endless numbers of combinations....and the SPECS don't tell you EVERYTHING you need to know....