With or without subwoofers, the process is much the same: one plugs the supplied microphone into the relevant socket and follows the printed instructions and hints on the display, taking measurements at the listening position and near each speaker. The unit then computes the EQ requirements to achieve a flat overall response and stores the parameters.

In use, one has the very sensible option of selecting low frequency correction (subwoofers only), low and mid (up to 1kHz) or low, mid and high frequencies, the highs being notoriously difficult to correct sensibly. It's all fascinating stuff and certainly a lot cheaper and more flexible than early implementations of such schemes – 15 years ago, the Marantz Audio computer cost £10,000!

The HD 990 has two features to add to the usual CD player set: digital inputs (to use its high-grade converter as an upgrade DAC) and a 'High Resolution Synchronisation' (HRS) link socket which connects to a similar one on the HK 990. This is a good idea that's been used before, sending a clock from converter to transport , so that jitter is no longer a critical issue in the digital interface.

Both the HD and the HK 990 feature HK's own digital filtering implementation 'RLS iV', and both have a balanced analogue connection alongside regular phono sockets, giving a choice of fiveways to connect the two: unbalanced or balanced analogue, optical or electrical S/PDIF and HRS. No such complications apply to the phono, which handles both types of cartridge.

Sound quality

With so many options on offer it seemed sane to start on familiar territory with the HD 990 connected via analogue leads to the HK 990, which was set to bypass everything possible. Under these circumstances we were first pleased, then increasingly impressed. Pleased, that is, that the sound obviously met basic criteria for quality, with decent bass and treble extension, neutral midrange, lack of obvious noise etc.

Impressed, because as we sat through various more or less familiar tracks from a variety of discs we realised that this duo is actually very capable and very musical. Those two things don't always go hand in hand. We've all heard products that seem to tick all the hi-fi boxes but still don't quite convince musically. In this case, though, everything comes together in a very happy manner to give performance that has scale, presence, imaging, detail – but above all, credibility and involvement.

You don't have to listen hard to this kit, the sound draws you in and gets into the brain with minimal listener input. And it does this, as far as we could discover, with pretty much any style of music one could name, everything from thrash to classical to folk-rock to Jake Thackray.

If there's a limitation, it's in dynamics which – perhaps a little perversely, given the power on tap – seem a little tame at times. Of course the amp will play loud, very loud indeed if one wants, but just occasionally we found ourselves wondering whether the difference between loud and quiet sections of a track were quite as wide as we'd heard on past occasions.

Taking things in small stages, we tried the various alternative CD-amp connection options. Quite frankly, we were struggling to hear differences. The balanced analogue connection seemed a shade clearer than unbalanced, with perhaps slightly deeper stereo images, but we wouldn't advise anyone to fret over it.

All three digital connections seemed broadly similar to the analogue ones, which should be a big surprise since, as far as we can see, all of them work properly and the D-A conversion in amp and CD player is carried out the same way. We did run some lab tests, which simply showed that HRS works fine, but doesn't offer much if any upgrade over S/PDIF because that works fine too, with next to no detectable jitter.