With your speakers, they're new enough that the crossovers are one of the last things to check as a possible culprit. The Solens caps won't have gone bad by this time; polypropylene caps are much hydrophobic than the older polyester film types that used to be used, which makes them more resilient. Even then, those old film caps tend not to go bad until they've had a good four decades on them in my experience. Unless the resistors and/or inductors have gotten toasty (or crumbled as some old sandcast types sometimes do), they should also be in spec. There is the possibility that there's a loose connection or cracked solder joint somewhere causing an increase in resistance on the woofer leg of the crossover. That is a possibility and something I've encountered often enough, but save that for later to look for.
?
For trouble shooting the reduced bass, you've already taken some good steps to that end and have received good advice. The first step in such a situation is to swap the speaker wires at the amp side of things. That's the easiest and quickest way to determine if the fault lies with the electronics. If it follows the channel, there you go. It'll also help negate the possibility of a slight short in the speaker leads at the amp side of things. A stray strand from the speaker wires shorting to the other post can lead to the amplifier still operating at a much reduced output without tripping the protection circuitry if it's not making too good of contact. Swapping wires will remove said strand if there is one.
?
If you've cleared the electronics, then it's to the speakers...maybe. Much as we put well-deserved weight of sound quality to the speakers, ultimately, the factor with the greatest impact on the sound is the room especially in the bass frequencies below the room's Schroeder frequency. If one speaker is in a corner and the other more out into the room or one ordinarily placed and the other near an opening into an adjacent room, there will be a significant imbalance in levels between the channels especially in the bass frequencies. It may be placement is causing your issues or you may be experiencing something related to more complex modal behavior. (50Hz falls in the range of common room mode issues due to the floor-to-ceiling distance.) You can cross this off your list of possibilities by swapping speakers, take the right channel and substitute it for the left and vice versa. If the depressed bass follows the speaker, you know it's not the room, but the speaker. If not, you'll need to work with placement to see if you can adjust for things using a SPL meter (even a basic SPL meter app for your phone will be a start) to suss out the room's behavior or progress to more in depth understanding using software like Room EQ Wizard and a calibrated mic to run frequency sweeps.
?
If it follows the speaker, you'll need to check the drivers. It could be the woofer in the one has loosened up over the years allowing for an air leak around the frame, which will reduce bass output. Beyond that, the DQ-20i is of the last series to still use the Advent style woofer, which is very prone to foam rot. All surround materials suffer age related decay, foam is just more visually obvious in it. Pull the grill covers and give it a good inspection. If pieces of the surround are missing, well, there you go. If not, check it more closely. Foam that is about to fall apart, but not there yet can develop slits that'll leak be it across the surround or at the edges where it meets the cone and/or frame. You'll have an idea it's getting to that point just by the color (foam in its last days takes on a yellowish tint) and especially by feel: run a finger lightly along it and foam about to give out will feel rough, crumbly and leave sand like grit on your finger tip with a pass. If it's at this point, plan on refoaming both woofers. This is easy to do with a kit from an outfit like Simply Speakers that offers a complete kit and instructions good for a first time refoamer, but there are plenty of other sources for foams if you feel more secure in going about it.
?
If everything checks out, pull the woofers. Measure the DC resistance of the voice coil and compare. A woofer that's been overdriven, but not to the point to blowing, can toast the VC enough to increase resistance. This will, of course, reduce output (and shift the crossover point). If that checks out, feel if the cone moves freely by applying gentle, even force around the edges of the dust cap and feel for resistance, grinding or hear a hashing sound. If you feel any of that, it may be the woofer has been bottomed out at some point, deforming the voice coil former causing binding that'll reduce output. Though, if that's the case, you should've heard a hash like sound before during playback. If everything checks out still, swap the woofers between the cabinets and see if the problem follows the woofer or sticks to the speaker.
?
If it doesn't follow the woofer, well, you've eliminated most everything but the crossover. As I mentioned, the caps are unlikely to have gone bad and if they've degraded, it's not to a degree inexpensive cap testers bought online can tell. They're garbage. I've seen one say an electrolytic cap was good that when dissected had no electrolyte left in it and was just a crystalized mess inside (ie as bad as it gets without having blown its guts out all over). Waste of money unless spending several hundred or thousand on an actually decent tester. More importantly, there is no cap directly in the signal path to the woofer. The 68?F cap in the woofer circuit is part of a Zobel network for impedance smoothing. If it were bad, the behavior around the intended crossover point would change and shift, but you wouldn't be hearing a difference at 50Hz. So, anyways, you'll need to eliminate everything else first. Check the connections, are the wires making good connection to the woofer? How about at the crossover? Between the crossover and binding posts on the back? Soldier joints look good on the board (ie no visible cracks when you look at them with a handheld magnifier)? I doubt you'll get to this point, having found the problem long beforehand, but this is the last likly cause of the issue you're having.
?
Good luck!
