combine the effects into a single filter.
<filter id="both">
<feComponentTransfer result="brightness">
<feFuncR type="linear" slope=".3"/>
<feFuncG type="linear" slope=".3"/>
<feFuncB type="linear" slope=".3"/>
</feComponentTransfer>
<feColorMatrix type="matrix" in="brightness" values="0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0 0 0 1 0"/>
</filter>