Hi, no problems - my JLH 1969 is seeing daily use and continues to work fine driving high frequency drivers in an active crossover setup.
I can't say.. I've filled my house with DIY audio gear and don't get out to listen to other stuff, so I don't have a reference point, plus I rarely run it full range.
I use mine with very high sensitivity, high frequency only drivers, but while testing I had it connected to DSE bookshelf speakers of about 86 dB sensitivity and it was OK for low/medium volumes. So I think it would be OK for nearfield use with 90 dB speakers.
I have seen online what appears to be a "cut down" version of this amp sold as a headphone amp. I haven't tried myself.