Which is better: Spendor BC1, SP1/2, or SP2/3.

What do you mean by “add”? You mean as alternative speakers to rotate with your current Spendors? Or to have four speakers playing at the same time?

The Spendor BC1 and B&W DM2 /DM2a are both very good.Which is better would really depend on your room interaction.From memory the BC1 probably has better imaging but the DM2 better power handling.Both use the Celestion HF1300 not Goodmans tweeters.The Rogers Export Monitors and Studio 1s are also similar.I have owned all of them as well as DM4s and Tangent RS4sThey are all good.

I've owned both the SP1/2E SP 3/1 speakers. I greatly preferred the 1/2E. Note that sweet spot of the classic Spendors is very small, but vocals do sound wonderful if you sit in the right spot.

For your room, given you have SP3/5s, I think the SP2/3 would be a nice fit. I have SP2/2 and heard the 3 a few times to know they have better bass control and more detail.

The SP1/2 and the SP1/2R2 are different animals. R2 is brighter.

Has anyone compared the BC1 with the B&W DM2 from the same era?

Both have the same top end with Goodmans tweeter and Coles super-tweeter but the bass/mids driver differs as well as the cabinet which in the DM2 is an acoustic line design whereas in the BC1 is ported.

Personnaly, i've just bought a pair of BC1, at first, i tought too much base and unprecise, than i've built a speaker stand with rosewood/pine/rosewood with pike.

Then magical appear, it was/is like the band is in my place, never heard a warm speaker like that before, except in the 80's when i was a student. I've own a pair of Tannoy mercury than a Tannoy Jupiter. There's something spécial in those speaker that took me over 20 years to find back! But what a result!

BC1 defetely an absolute, espacialy if you enjoy small band music(Jazz or Folk).

If you can get mint BC1s, they had a magical sound that was somewhat lost with the following iterations, SP1s and the rest. They also had a slightly underdamped bass that modern amplifiers would have no problem with. Lots of warm bass and clarity but volume limited.

My experience is with the smaller, more WAF Spendors, wherein I'll recommend that you chase the Classic S3/1p, which is like the 3/5 except that with a vented 6.5", and therefore has much greater extension, while also being +4dB more efficient. I use a pair of these along with the excellent SC3 (like the 3/5 but with a second mid-woofer) to make a seamless HT front trio. Forgiving up top, extremely natural mids. I use a sub in this system, so the larger Spendors would be inappropriate.
The Boston Spendor dealer still loves the SP1/3, and sells them to us old farts who can accept the archaic box geometry!