well I have a UA LA-610, which is a UA 610 preamp combined with an opto compressor, similiar to a UA LA-2A. It is a fully valve unit whereas the 6176 is valve on the preamp and solid state on the compressor. I also have some DIY UA 1176 clones which are not mic preamps but compressors. It is a little hard to compare the ISA828 and the 610 section of the UA6176 or UA LA-610 as they are so different. The UA 610 preamp I have in the LA-610 is very warm and distorts very easily, it also doesn't have a great deal of headroom, when combined with the valve opto compressor it provides a very warm and rich in harmonic content tone if set correctly, but can be very difficult to work with if the singer has a wide dynamic range and you don't want to get obvious clipping, but still want to be in the "warm range" On the other hand I believe the UA6176 is easier to work with while still capable of quite high levels of distortion. The FET type compressors in the 6176 and 1176 units are capable of much more transparent or much more extreme compression without sounding overly distorted than the opto compressor in the la-610. On vocals the 6176 is very good for female vocals and probably most other vocals. The 828 is also very good and when combined with a 1176 compressor would sound similar to the 6176 on a cleaner setting, but would probably be cleaner again without sounding thin. It has a really nice silky sound on vocals when combined with the 1176. If I had a choice of one or the other, 828 versus 6176 for vocals and that was the only choice I would probably go with the 6176. As I have 6 x 1176 type compressors and do a lot of multi instrument recording the 828 is very practical.
Also the regarding the la-610 it is great for very warm vocal sounds especially mid to deeper and less delicate voices.
I hope that helps, please let me know if you have some other questions.
