It definitely sounds as though your wood may not be premium, oak can typically take the 18-24 mlnths like you stated. I had some oak rounds 2 1/2 years old i finally split and it was still at 26%-30% moisture. Once you have gotten the stove in that 400° range have you tried to close the air down to half or maybe less to see if the secondaries will continue. These stoves seem to get hotter with the air closed allowing the secondary burn tubes to create all the heat apposed to having the wood burning.
Have you tried a couple biobricks and a couple splits to see if the ultra dry bricks would compensate for the possible wet wood?
I didn't notice if you mentioned that you had a block off plate or if you have insulated the fireplace behind the stove, these are key things to keep the masonary fireplace from soaking your heat up and releasing it to the outside. I feel my stove gets up to higher temps because of all the insulation I have packed behind/around it. The surround lifts right off so you can visually see. If you could maybe post a pic or 2 behind the stove, it might lead us to a more definitive answer. My guess is that you're losing the heat there.
I haven't found many clydesdale burners unhappy with its operation unless their wood was subpar on all the searches I've done. This was not my first choice in stoves as I more settled on, but now am very happy with it. Originally was going to get the BK Princess but could not be installed with my chimney

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I had mine walk away from me this morning, I loaded N/S about 10 splits of oak and maple and she got up to 690° with the fan on high and the window open next to the stove. With it still climbing I tried the open door trick, but ended up having to throw a wet frozen round on top to get the temps back down, she dropped for over an hour. Been burning the coals all day so far. I jus got my flashing buzzer alarms I am going to wire one downstairs and one upstairs as an early warning system.
FirewoodAddict you say you run your stove 400°-700° on a regular, have you seen any ill effects from the super heat.... having mine only hit over 700° once or twice I have not seen anything that looks warped or over heated. For the cost of these stoves I'd hate to have to tear it apart to replace a panel on it ha