I just sold my BK-9. Luckily, somebody was keen on a lightweight 76-key board with organ drawbars and paid 90% of my purchase price of December.

Although I still have mixed feelings about the sale, as it was a unique arranger in the sense that it was low-weight with 76 good-quality keys and without built-in speakers (those speakers are a no-go for me), I think it was the right decision in view of the cons I lately realized and the fact that I already have an Integra-7 soundwise.

There is one problem about the BK-9 I only realized last week, it may be a detail only for some users, but essential for me, as I often need pure piano plus strings:

The BK-9 clearly has a polyphony problem.
When you play a SuperNatural piano and add the best strings to it (Full Sect.2), notes are quickly cut off when you play with sustain pedal. Not just when making excessive use of it, but with normal playing of a pop ballad. It is very striking.
I am sure the cause consists of the following two factors:

1) The SN-A sounds require much power from the processor, so they eat up much more polyphony than an ordinary PCM sound. (Roland mentions this with respect to the Jupiters and Integra.)
2) The strings "Full Sect. 2" appear as one tone in the BK-9, but from the SRX 04 expansion board from which it stems, I know that the original patch consists of 4 waveforms. Even if you don't see these 4 components, they are there and eat up polyphony.

I still have to check if the same combination of sounds generates the same problem in the Integra - so far I have had very few polyphony problems with the Integra. If the problem doesn't appear, this means that the BK-9 processor is too weak to handle SuperNatural sounds in combination with other sounds.

You may ask: then why not just take a different strings tone that doesn't require so much polyphony? - That's my other criticism: This one strings tone is excellent; ALL the others are average to low quality. Why didn't Roland add at least a handful of different SRX 04 strings to the BK-9? Instead, there are 20 or more strings of a rather low quality, taken from the BK-7m and older instruments like GW7.