The SPR-4 is an interesting Drake. Some refer to it as "the solid state R-4B" since it continued to use L/C filtering instead of the crystal IF filters used in the R-4C but it does use the newer PTO style of the C line.
The sensitivity and stability are excellent. The only thing I miss from the R-4B is pass band tuning and it may have been left out in order to keep the SPR-4 fairly simple to operate. The L/C filter bandwidths are well chosen for AM, SSB, and CW and it sounds good on all modes. The noise blanker and calibrator are optional and it seems a lot of those up for sale don't have one or both of these options. It does have a built in speaker.
Drake referred to it as programmable in some of their advertising because you could buy different sets of range crystals (shortwave broadcast, amateur, tropical bands, etc.) based upon your interest and they came with decals to label the switch. I believe the range switch was originally built for the CB market of the time and it provides 23 selectable ranges each covering 500 khz. There are some used SPR-4 receivers floating around that were former FCC units and the CB channels are marked directly on the PTO dial.
Mine came with the NB and calibrator and both work as expected. I built one of the "crystal eliminators" from the Hageman QST article to provide complete .2 to 30 mhz. coverage for my SPR-4. It works well and I bought two more kits for the regular 4 line gear. Note that the range crystal frequencies are different for the R-4 line and SPR-4 line so crystals for one will not provide the proper range in the other unit (regular 4 line crystals will create a 10 Khz. calibration error); in the case of the "crystal eliminator" you had to specify when you ordered the kit. I don't know if the programmed parts and board are still available but here is Hageman's website: http://www.oocities.org/hagtronics/r4.html
Of course you can also buy the Drake FS-4 full coverage synthesizer but they tend to be collector priced and slight modification is needed for operation with the SPR-4.
I have a Drake AL-4 loop that came with my SW-4A receiver and it is a useful accessory for the broadcast band (the jack it plugs into is only active through 1.5 mhz. so the loop doesn't function on the higher ranges). If you want to build your own instead of paying a high price here is a link to a construction diagram: http://www.dproducts.be/drake_museum/im ... _to_do.jpg
There is a rubber piece under the tuning knob in the C-line style PTO (includes the SPR-4) that will cause the PTO to constantly lose calibration when it is worn. You can replace this piece easily if that is the problem. It is also a good idea to clean/lubricate the PTO to prevent wear and preserve smooth operation and several internet sites provide a good explanation of this process. Do NOT get lube on the plastic drive gears. The Drake PTO is very easy to work on.
Let me know if you have other questions. Your SPR-4 is a fine receiver when operating properly and is well worth getting back into proper shape.