I can finally lay this autopsy of the SP2500 to rest. It is not a bad design and clearly there are Klipsch and JBL influences here. There are even rumors that the latter were involved with the design and JBL and Sansui did have a joint venture during this period so it is feasible. First, the box itself. It is not too small. It is just built from the port sizes up (2.5"x3.125"). The box is rated 50-20Khz. So using 50 Hz it is not too hard to calculate the end correction factor for the ports. From there we can calculate box size to approx. 1.3 cu. ft., which just happens to be the size of the internal enclosure itself. So, that mystery is solved. As for the crossover, Sansui or whomever was more interested in acoustic effect (think Kabuki Bose) rather than electrical so we are not going to find the predicted values of the crossover components present on these boards. This became clear when measuring SPL plotting by hand. There are cost cutting measures present that was put into this theory as well. Example, the specs state the crossovers are at 700/6500. The former is probably some acoustic number and the 6500 an average based on the tapped tweeter inductor. The bottom number is impossible to achieve using an electrical response with the original woofer or any replacement woofer. The midrange is just choked up too much and the woofer to midrange has an intentional exaggerated overlap as a result. Thus, the crossover network is really just for the twin midranges and twin tweeters both in parallel. Interesting, and accoustic in nature again, the tweeters and midranges are connected in reverse polarity (same effect as just reversing polarity at the woofer). Tweaking the tone knobs only increase or decrease the left slope of the midranges and tweeters. The higher frequency loudness can be a little too much to handle with today's playback media. But there is a cheap solution to bringing these boxes up to date and playing very well without fatigue.
My SP2500 tweak is cheap and consists with starting with the replacement woofer. 8 ohm is the most abundant out there so forget about finding a new 6 ohm 12" woofer on a budget. Mine are a couple of Pioneer badged 12" from Parts Express. The woofer needs just a little filtering so I used an undersize 1.0 mH inductor and normal size15 uF capacitor for a 2nd order at the woofer. The inductor was mounted in the box and the capacitor was mounted in-line. The type of woofer doesn't really matter just make sure it can be used in a sealed or ported box as far as specs go. I also added a Zobel to even out the impendence. I won't go into capacitor replacement on the crossover because it has been beaten to death but it is something that needs to be done. I used some of the better caps but in hindsight they might just be too good for these speakers. You may just end up with more high frequency SPL than you intend to get. The midranges and tweeters are that good. If I had to do it over again I would have just went with the $1 caps (same thing with the Zobel). The midranges and tweeters are no joke and they may need to be choked down -3dB with L-Pads. With my 80W HK amp at about 15 watts output I'm getting an average acoustic SPL of 93 dB with a standard deviation of less than 6 dB (36 plots measured between 40-10khz) in my listening room and will pick up 19kHZ at 65 dB. Sounds most excellent now. Kabuki Heresy III territory. Don't expect spectacular bass below 50Hz. It is just not going to happen and Sanui is relying more on perceived lower response as most commercial manufacturers do to this day. There is some good science on that topic but this isn't the place.
L-Pad for midrange resistors I used are 2S/12.5P. The tweeters were 2S/16P. L-Pads were connected in-line and stapled to the rear panel board. Zobel cap and resistor will depend on individual woofer specs. Mine were placed right at the woofer terminals. So, I never made a network component board for any tweaks.
I think with a couple of MCM 55-2952 woofers and economy capacitors and resistors the whole thing could be less than a $60 upgrade and big improvement.