hi,
i would like to highlight the following points in seismic analysis based on my experience.

1)Generally multi point response spectrum should yield low stress results,compared to envelope spectrum.)

2) IF you are going to consider seismic anchor movement(SAM),it induces secondary stress so allowable limit is very high compared to inertia loads.(check the code allowable limit for SAM)

3) Do multi point response spectrum analysis by setting SAM as Zero and check the results,if still exceeds the limit compared to enveloped spectrum kindly check the grouping of spectrum .Try a piping benchmark problem in Nureg report (where they have compared results for envelope and multi point response spectrum method for identical model)

regards
Udayakumar