Baking below sea level
Your altitude won't matter in your baking. Whatever reason your cakes have for not rising, it's not that you are a tiny bit below sea level.
First of all, some personal observations on altitude: I've baked frequently at about 750 meters (let's round it to 2500 feet) and also in my current home at about 130 meters (about 425 feet). The same recipes work without noticeable differences in rising. A reduction of over 2000 feet doesn't bring much visible difference, much less the difference between &quot;working perfectly&quot; and &quot;falling&quot;. Even if your recipes were perfected for 0 feet (which they aren't, cooks don't do it that precisely), a reduction of only 600 feet will be unnoticeable.
Second, what could the theoretical effect on rising be? Well, the air has less pressure due to altitude, so you'd expect slightly higher density; the thousands of air bubbles in the cake won't expand as much. How much is this &quot;slightly&quot;? Don't forget, air density depends on both pressure and temperature. For comparison, I calculated the air density at different heights and temperatures: It is the same density if your air is at 600 ft below sea level, heated to 175 Celsius (a common cake baking temperature), or sea level at 165 Celsius.
Now, our ovens have nice knobs graded in small intervals, but they are actually very bad at keeping temperature. It's not only that their mean temperature is badly adjusted (I've frequently seen ovens running 40 Celsius low), but also that their temperature fluctuates all the time with an amplitude much higher than these paltry 10 Celsius. And this is only the temperature outside of the cake; the air inside the cake (which does the expanding) never reaches 175 Celsius. A cake is ready at 100 Celsius internal temperature at the latest (all starches have gelated by then), frequently below that.
So, to summarize: A cake's height will change a lot more during a bake due to random factors than due to your altitude. Even if you had a perfectly controlled process, you'd need vernier calipers to measure the difference in height. But when using an only reasonably controlled process at home (ingredients at room temperature, measuring weight with a digital scale, controlling oven temperature with a thermometer), you will already see fluctuations larger than the effect of 600 ft altitude difference.
Appendix
My calculation (formula and gas constant from Wikipedia, air pressure at different heights from engineeringtoolbox):