I've done long rides on my Nishiki Cascade (mostly on the road, 50, 65, 75, 100+ milers). A few of the earlier rides were on knobblies. I didn't realise just how noisy they were until I changed the tyres for knobblies with the hard ridge down the centre - inflated to 85psi they were great in a straight line, but still noisy on the corners.
Halfway through last season, I put some 1.5" semi-slicks on and the difference was amazing! So quiet, so smooth - but still much better off-road than a skinny-wheeled bike. At the end of the season, I started using the MTB on the rollers and trainer, and even the semi-slicks were pretty noisy, so I changed to completely slick tyres which, at 85psi are as quiet as quiet can be! If you're doing lots of mixed riding, I think the slickies are pretty good, even for moderate off-road riding.
Riding the MTB for long periods was quite uncomfortable with the supplied straight bars - so I changed them to "butterfly" or "trekking" bars - I did a lot of tweaking of the stem and angle of the bars to get them just right (see other threads for pics) - I think that, and the tyres made a vast difference to long ride comfort on the MTB.
I've also tweaked the gears over the winter - from (46-36-24 and 12-14-16-18-21-24-28), to (50-40-32 and 11-13-15-18-22-26-34)
- Wil