My speakers are probably twenty five years, (+), old. I've already played about with them by removing the passive radiators
Seems like B&W agreed with you because they replaced the passive radiator with a powered drive unit in subsequent versions (i.e. S2 & S3) to make a 2.5 way speaker - and now it seems the current B&W 603 is a three-way with two powered bass drivers.
I have thought about buying some drivers to replace the passives, but understand, (from reading), that this could get a bit complex and so I backed off, although I'm interested to hear others views on this.
It's true that replacing a passive radiator with a powered drive unit is experimental and not entirely predictable because there are so many variables. I guess it depends how strong the urge to tinker is (surely we all get that feeling from time to time?). Good thing about adding a powered driver is that the changes made could be easily undone and the speaker returned to it's current build.
Incidentally, is the passive radiator hole in the cabinet blocked/sealed up or have you left the cutout hole open?
There are a few things to be considered - though there are a good few lesser factors too...
Check the nominal impedance of the speaker 'as is'. If it's currently 6 Ohms or less it's probably unwise to attempt the modification because the added bass drive unit would likely lower the modified speaker's nominal impedance to below 4 Ohms which most amps/receivers won't appreciate (though a few will cope admirably). The chosen bass driver should have a nominal impedance of at least 8 Ohms.
The existing crossover probably won't need altering but a secondary low-pass filter to feed bass signals to the new bass driver will be required. The new secondary low-pass filter (with a centre frequency in the region of 200Hz to 300Hz - typically, determination of the optimum crossover freq. for a particular application would require measurements and trial-and-error) needs to be connected in parallel to the existing bass/midrange driver. This will turn the speaker into a 2.5 way speaker where both the existing mid/bass driver and the new bass driver will share bass duties. Doing this mod. will lower the speakers impedance in the bass region but assuming the DM603 is an 8 Ohm speaker and you add a bass speaker with 8 Ohm impedance (or greater) and your amp/receiver is happy to drive a 4 Ohm load, then all should be well. This example is the type of filter required (x2) although filter (centre) freq. seems a bit too high at 350Hz. Nearer 200Hz - if there's one out there - would have less sonic impact on the rest of the speaker:
Maximum power: 300W. This subwoofer-dedicated crossover is designed for passive subwoofers. +W = bass positive. -W = bass negative. Frequency points: 350Hz. IN = audio input.
Some, possibly many, 2.5 way speaker designs have a separate enclosure for the dedicated bass driver built into the cabinet so each driver is presented with the optimum loading. I suspect that if you chose a dedicated bass driver that has a similar efficiency (or lower) compared with the current speaker and is of similar size, this won't be too important. However, some bass speakers are designed for sealed box and some for reflex port whilst others are suitable for either type of bass loading. As a generalization, bass speakers with a lower resonant frequency usually suit sealed cabinets, like the DM603.
Personally, as this exercise would be an experiment, I wouldn't spend big on a dedicated bass driver. Worth looking at Electromarket who sell some half-decent cheap bass drivers, such as:
Rubber suspensionPolypropylene coneSpeaker size: 6.5 inch (16cm)
125W RMS Power/250W Max Power6.5 inch kevlar reinforced speaker cone with rubber roll surround and carbon weave finish in yellow and black125W RMS with vented magnet and 4 layer voice coil for superb power handling and performanceBlack metal chassis with 30oz - 120mm diameter vented...
The above drive units are examples and not recommendations.
And there's always ebay sellers. It's likely that some woodworking will be required as different drivers (even ones of the same nominal size) often have different cutout hole sizes and fixing requirements.
'Fraid I haven't a clue.Regarding the capacitors, would it be worth a change? Or is it possible to test the existing capacitors to understand their condition?