Single driver full range speakers

I have a pair of PearlAcoustics Sibelius single drivers speakers. Cabinet design is CRUCIAL: in this case a Voight tube. My personal preference is low clean bass, no window rattling onslaught. The Sibelius itself is perfectly fine (38 Hz, with 32–36 Hz -6 dB), but still added a REL T9 set next to one speaker. It is set at very low volume, don't remember cut-off (crossover) frequency. 

I love the coherence a single driver produces. The low bass has very little directionality, so a separate sub works fine. 

Many years ago I heard a pair of Ocellis speakers (don't recall the model) with the PHY driver. A high-efficiency speaker that sounded very natural and full of life. Don't hear much about them anymore. 

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.