Single driver full range speakers

Cabinet design and amp are also factors in bass production. I have a set of full range speakers that I had connected to a Marantz model 8. The bass was thin and I had to use a sub to compensate. I started running them on another tube amp and now I’ve got bass in spades.

A couple of things to consider:

Adding a driver will affect the impedance loading on the amplifier, increasing or decreasing depending if wired in series or parallel. 

The published (or expected) frequency response of a speaker is different than the in room response. A speaker's in room low end is often ten or fifteen dB higher than published specs taken in test chamber. So even if there was a way, realistically how much of a benefit would it be.

You could try a passive radiator or a down-firing port but that takes as much design horsepower as crossovers since you have to take the effect of the cabinet on the full range driver's performance.

Or, you can try a simple first order inductor.

@OP. The drive unit in question not only has a rapidly falling frequency response below 150hz, but it has a strongly rising response above 1.5k - and a ragged one that's typical of whizzer cone "full range" drive units. The latter will exacerbate the sense that the speaker has no bass. The problem with adding a bass unit is that you will have to match sensitivity as well as frequency response. That will be nigh on impossible to do without using a crossover. So you would be better using a sub. At a philosophical level you will still be able to view your speaker as having no crossover - since that will be in the sub.

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:

1. 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.  
2. Port tuning:  If the existing cabinet is ported, the port tuning may change, causing bass to be overly ripe and less detailed.  
3. Cabinet resonances may be produced or existing resonances amplified by adding the woofer, degrading sound quality. 
4. 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.  
5. 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.  

Thank you all. You've made me feel/realize I am getting as much as can be expected out of these DIY speakers. I really do love their soundstage and presence. I do have dual subs as noted and they mate beautifully. I was just wondering if I'm missing anything as I'm relatively new to single driver magic.

@total111  Thanks, I'll take a look at the the Tocaro speakers you mentioned.

Thanks all again.