For maximum comfort, you'll want to maximize the air volume of your tires. It helps to minimize the sidewall stiffness, too. Knobs (or thick thread) take up valuable space that could otherwise be used for air. Knobs are used to increase traction on sand or mud; for dry gravel, they don't improve traction or comfort. Most of the 26 x 1.9 to 2.0" tires seem to have really thick, "durable" treads and stiff sidewalls. Your best bet might be a set of MTB XC tires with the knobs trimmed. Other options could include Vredestein Spotted Cat, Schwalbe Marathon Mondial, Schwalbe Big Ben, Schwalbe Jet, Panaracer Ribmo, Schwalbe Kojak. I'd caution against the Vee Speedster which otherwise looks good for your application -- we found that the tread picked up lots of glass and small rocks compared to every other tire we've owned.