I simply put a series switch with the bass speaker. For some reason the system now keeps cutting out and I have no idea what for.
That's unusual... Most power amps are happy with no load.
Do you know the speaker impedance?
But I have read that this will need to be a huge resistor because of the amount of power the speaker is. (it is a 25w sub woofer)
You can get high-power resistors, but I might be hard to find a 4 or 8 ohm resistor. [u]Here[/u] is a 5 Ohm 25 Resistor for about $3 USD. Or, you can put resistors in series/parallel to get 4 or 8 ohms. But since speaker impedance varies over the frequency range, the rated impedance is just the "nominal" impedance.
Some random thoughts...
You shouldn't need the exact-same resistance (impedance) as your woofer. I think a higher value should work. If the resistance value is too high, It will act like no resistor at all (or maybe about the same as your woofer with a series resistor). But, a higher value resistance will also draw less current and therefore dissipate less power & generate less heat.
If the resistance is too low, too much current (and power) will be "pulled" from the amp, and you could damage the amp.
. (it is a 25w sub woofer)
You probably don't need a 25W resistor, but it's hard to say exactly what you do need. Regular audio has peaks that are typically 10 times the average. So, unless you are running full-power test-tones into your speaker, it's not seeing 25W continuously. If you have the volume maxed-out (as high as you can go without distortion) you are probably averaging around 2.5W. Less, if you are listening at lower volumes. But to be safe, I'd probably try a 10W resistor. It also depends on the music, and you can get the full amplifier-power continuously if you drive it hard into distortion. But, if you can double the resistance (compared with the woofer) and the thing still works, you'll get half the current through the resistor, which means half the power.
...which is enough for my parents to shout up stairs.
If your sub is sitting on the floor, moving it to a shelf (or somewhere off the floor) should reduce sound transmission through the floor.