I am not sure Arv. The real solution here is like a "fork lift" upgrade, replacing everything after T11 in the transmit chain. If I were doing a redesign I would use the same four filters, add five more relays, and decode the three lines to get four lines each driving the pair of relays bracketing the desired filter.
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I would also use latching relays and move the TR switching to the PA side of the filter bank to allow the filters to be utilized on receive as well.
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All relatively easy to do...... just takes time and money. And motivation.......
WA8TOD