This is uncanny!... This morning I fetched from my cupboard, amongst other drivers, a pair of KEF B110 SP1003 and four Monacor SP-135TC with voice coils wired together to give 16 ohms.
I bought the Monacors several years ago to replace the four Audax HD13 drivers in one of my pairs of JR150 but have not yet attempted to fit them as apparently there is some jiggery pokery required with standoffs to fit the driver proud of the baffle as its basket is fractionally larger than that of the Audax HB13.
I took measurements of all drivers this afternoon. The measurements were done in free-space with no baffle or enclosure so are of limited use, but may be of some value nevertheless. I took them at a distance of 1m and off-axis by about 0.4m (it's been a long-time since I did trig so I'll leave it to someone else to calculate the angle from that! I measured off-axis because I was measuring tweeters as well and those exhibited a sharp dip at 6kHz when measured on-axis.
I'm surprised you don't notice a difference in volume level between the B110 SP1003 and 16 ohm Monacor SP135TC because, according to my free space measurements, there's an efficiency difference between the two drivers that amounts to 3dB below 1kHz and 8dB above 1.5kHz?! I also understand that the JR149 has an "anti-quack" notch filter to tame the B110's response at the upper end of its operating range, which I suspect will produce sub-optimal results when used with the SP135TC unit. I'd be very interested to see measurements, if you have taken any? Also, may I ask why you decided to use the SP135TC in 16 ohm configuration in the JR149, wouldn't 8 ohm be more appropriate?