There are too many technical considerations and variables for a positive recommendation to modify your existing single driver speaker cabinets to accommodate a separate woofer without a crossover. While I recognize you built the existing speakers yourself, you need to consider the following before proceeding:
- Cabinet volume and pressurization: If the volume of the cabinet is inadequate, the back wave produced by the added woofer may over pressurize the cabinet presenting a potential to damage the woofer or original speaker, or affect the performance of the original speaker, causing sound quality to be vailed.
- Port tuning: If the existing cabinet is ported, the port tuning may change, causing bass to be overly ripe and less detailed.
- Cabinet resonances may be produced or existing resonances amplified by adding the woofer, degrading sound quality.
- Cancellation and reinforcement: Without a crossover, the new woofer will operate in an overlapping frequency range as the existing single driver, reinforcing some frequencies. Also, this may cause cancellation effects. The frequency balance and coherence single drivers are known for may be negatively affected.
- Time alignment and phasing: Unless you are able to allign the woofer with the existing single drivers without, you may introduce phase distortion.
Alternatively, I recommend adding a modern powered subwoofer(s) in parallel, without using an active crossover with your speakers, rather using the filters most subwoofers have to roll in gently at the frequency you desire. You can time align with judicious placement and set volume and roll in to be coherent with the single driver. Also, modern multi driver speakers with crossovers are much improved, and are as coherent as single driver speakers, extending lower than most single drivers. I recommend auditioning some newer designs that will give you the bass response you are looking for.