This may depend on what the problem is. The HK990 is a good amplifier, but many early models did suffer from problems (related to the amps cutting out sporadically). This was fixed by HK in later models. However, this was probably down to a modification of some proprietary part, so it may be difficult for a general repair specialist to help. The HK990 also has a deal of proprietary software on board. If your problem is related to this then again repairs by non HK agents may be problematic.It's probably worth finding out how much an official HK service agent would charge. Ask HK for your nearest. A simple Google search revealed the following (not exactly West Midlands though!)
http://www.multicare.org.uk/repair-services/home-electronics-repairs/harman-kardon-home/